Concealment or "invisibility" with Minor Illusion?

[imneuromancer]

So let's say you are a character that fits within a 5' square. If you make an illusion of a metal box around you, what affect would that have on opponents trying to hit you?

"Physical interaction reveals it is an illusion, because things can pass through it." -- Spell description

Would the first attack a creature makes against you
a) have complete cover?
b) be disadvantage because they can't see you, similar to invisibility?
c) not be hampered by the illusion at all?

Obviously the illusion would not hamper area of effect attacks, but what about targeted spells/attacks?

What if you made the illusion of just a small area of darkness, which would make SENSE that items pass through it, would that immediately alert the attacker that it is an illusion? Would that modify the answer to the question of the first attack, above?

(BTW: I searched for a discussion on this topic, didn't see one. Please link if this has already been asked/answered.)

[JeffreyGator]

I believe that this is a standard use for minor illusion - creating a place to hide behind/in.

Create something to hide behind/in.

Even once something has been revealed to be an illusion, it takes an action and a successful investigation check to see through it.

[gfishfunk]

So let's say you are a character that fits within a 5' square. If you make an illusion of a metal box around you, what affect would that have on opponents trying to hit you?

"Physical interaction reveals it is an illusion, because things can pass through it." -- Spell description

Would the first attack a creature makes against you
a) have complete cover?
b) be disadvantage because they can't see you, similar to invisibility?
c) not be hampered by the illusion at all?

Obviously the illusion would not hamper area of effect attacks, but what about targeted spells/attacks?

What if you made the illusion of just a small area of darkness, which would make SENSE that items pass through it? Would that mean physical interaction would not reveal it as an illusion? Would that modify the answer to the question of the first attack, above?

(BTW: I searched for a discussion on this topic, didn't see one. Please link if this has already been asked/answered.)

I had a similar thought: advantage and disadvantage would be a wash: the attacker would get disadvantage for not seeing you, you would grant advantage because you can see the attacker.

I would rule it a whole different way: unless the attacker makes an investigation check, the attacker believes that you summoned a magical wall or something similar, and would have no reason to think s/he could attack through it.

EDIT: for all you jerk GMs out there, next time you have the enemy stupidly reach out to touch the illusion that just appeared (thereby proving it to be an illusion), make sure you do the EXACT SAME THING when a creature is summoned, or when a fire suddenly appears, or when the caster shoots out lightning.

Another alternative thought I had: summon a black box around the attacker's head. That way, you can see the attacker, but the attacker cannot see you. If the attacker moves out of the 5' by 5' headbox area, you get an attack of opportunity with advantage (s/he cannot see you). The attacker would have no rational reason to believe the box was just around the head, unless s/he saw that trick earlier, and may conclude that a darkness spell was just cast around them.

[Blue Lantern]

I would never give complete cover from that, cover suppose an environmental item that could physically block an attack, which an illusion would never be able to do.
Even disadvantage is dubious, considering that you are trying to hit a static box, not an invisible moving target of being in pitch black darkness.
All you would get would be blocking line of sight an give a chance to hide.

[imneuromancer]

I had a similar thought: advantage and disadvantage would be a wash: the attacker would get disadvantage for not seeing you, you would grant advantage because you can see the attacker.

The person casting the spell knows it is an illusion, so they would automatically see past it. "If a creature discerns the illusion for what it is, the illusion becomes faint to the creature." So the attacker cant see the illusionist, but the illusionist can see the attacker...

[imneuromancer]

Even disadvantage is dubious, considering that you are trying to hit a static box, not an invisible moving target of being in pitch black darkness.
All you would get would be blocking line of sight an give a chance to hide.

But you cannot see the person inside the box/darkness created by the illusion.

[Blue Lantern]

But you cannot see the person inside the box/darkness created by the illusion.

Yes because swinging a 6ft long greatsword you really risk missing someone inside a 5ft sized box. Not to mention that the second you touch the thing it becomes transparent.

[NecessaryWeevil]

Yes because swinging a 6ft long greatsword you really risk missing someone inside a 5ft sized box. Not to mention that the second you touch the thing it becomes transparent.

If it's that easy to hit a creature inside a 5ft box (which all medium creatures occupy in D&D), why do we have AC and attack rolls?

[Millstone85]

It is a shame minor illusion can't create a big exclamation mark and that sound (https://www.youtube.com/watch?v=2P5qbcRAXVk) at the same time. It would be perfect for when someone sees through your box disguise.


I would never give complete cover from that, cover suppose an environmental item that could physically block an attack, which an illusion would never be able to do.I agree.


Even disadvantage is dubious, considering that you are trying to hit a static box, not an invisible moving target of being in pitch black darkness.Well, I don't know if it is realistic, but you are supposed to have disadvantage against unseen targets even when you are not guessing their position.

[Blue Lantern]

It is a shame minor illusion can't create a big exclamation mark and that sound (https://www.youtube.com/watch?v=2P5qbcRAXVk) at the same time. It would be perfect for when someone sees through your box disguise.

Two levels of illusionist will let you do that.


Well, I don't know if it is realistic, but you are supposed to have disadvantage against unseen targets even when you are not guessing their position.

Maybe, but this is a situation where I think the rules need some adjustments, getting free advantage for 1 minute for an action is way too much.

[JackPhoenix]

Maybe, but this is a situation where I think the rules need some adjustments, getting free advantage for 1 minute for an action is way too much.

It's not for 1 minute, but until you attack, or someone try to hit that box. Remember, physical interaction (no matter which side of the illusion it came from) will reveal it to be fake. If the caster tries to attack from seemingly solid, he'll reveal it as fake to anyone looking. You'll have to use attack that don't come directly from the caster, Create Bonfire would work, as would Chill Touch and (based on description) Sacred Flame

Also, attacking the box is not jerk GM move: even if the wizard created a real box to hide in, you still want to kill him... the way to do it is to hit and break the box first. Except after the first attack, the box turn transparent due to physical interaction, and the only thing you can hit (with disadvantage) in its area is the caster. There's no actual substance to block the attack from going through.

[gfishfunk]

Maybe, but this is a situation where I think the rules need some adjustments, getting free advantage for 1 minute for an action is way too much.

Its fine. 1 Action, plus concentration, plus the likelihood of the illusion breaking due to inspection or interaction is not too bad.

The following approach is still the most rational: an enemy will conclude that the wall / ice shield / whatever is really there (just a conjured effect) and won't try to attack it. The enemy will either circle it, investigate it (potentially destroying the illusion) or go after someone else.

[Millstone85]

Even once something has been revealed to be an illusion, it takes an action and a successful investigation check to see through it.
It's not for 1 minute, but until you attack, or someone try to hit that box. Remember, physical interaction (no matter which side of the illusion it came from) will reveal it to be fake.I would go with JackPhoenix's interpretation of the cantrip over JeffreyGator's. So, it is a better version of true strike, something the game certainly needs.
Edit: What if you do it again? Should enemies immediately see through the illusion?

[Joe the Rat]

You use your action to make your opponent attack at Disadvantage.

So you're using a spell to achieve the same effect as the Dodge action? Works for me.

[Slipperychicken]

If the attacker is aware that minor illusion is a thing, and thinks you had cast an illusion, then he might try to shoot through it anyway. I'd resolve that as a disadvantaged attack roll. More likely, he might not bother and try shooting someone else instead.

Also, minor illusion doesn't do darkness. It gets you "a sound or an image of an object".

[SharkForce]

If the attacker is aware that minor illusion is a thing, and thinks you had cast an illusion, then he might try to shoot through it anyway. I'd resolve that as a disadvantaged attack roll. More likely, he might not bother and try shooting someone else instead.


i wouldn't expect them to try and attack through unless they've recently been subject to illusions, or are specifically trained to always expect illusionists rather than any other type of wizard (for example, if they live in a nation where the only school for magic trains exclusively illusionists).

of course, once they've seen the trick once, they should absolutely attack through it next time. and of course, once they've attacked through it, they should be able to see through it (still protects against one attack of course, and may give you advantage on your attack).

lastly, i don't think it always makes sense to avoid an object (whether you think it is an illusion or not). the wall of stone spell has rules for how hard it is to break; i have a hard time imagining a world where people might consider smashing a wall of stone but would never consider smashing through an ice or wood wall. of course, that still takes up an attack (and i would say there could potentially be a chance they'd still hit you), i'm just saying they shouldn't be standing there dumbfounded just because you are hiding in a box and they think they can't directly attack you.

[Malifice]

So let's say you are a character that fits within a 5' square. If you make an illusion of a metal box around you, what affect would that have on opponents trying to hit you?


They probably wouldnt swing at an iron box anyway (they'd probably try and find a way to open it).

If they did swing at it, then its total obscurement (disadvantage) for anyone inside.

[Blue Lantern]

You use your action to make your opponent attack at Disadvantage.

So you're using a spell to achieve the same effect as the Dodge action? Works for me.

No you are using a cantrip to achieve the same effect as the Dodge action for the entire team for 10 rounds. Quite the difference.

[JackPhoenix]

No you are using a cantrip to achieve the same effect as the Dodge action for the entire team for 10 rounds. Quite the difference.

IF the whole team fits into a 5' cube without touching it and doesn't do anything for those 10 rounds and the enemy doesn't investigate, attack or even touch the thing they are hiding in. That's... even less useful than bringing a real box and hiding in it, that one can at least stop few attacks.

Minor Illusion is useful for hiding, but pretty much useless in combat

[Dalebert]

How effective an illusion is depends entirely on how believable it is. Keep in mind that it's a cantrip and one of the best ones, so it's quite possibly the most common spell in the entire world. What kind of DC would you give for the arcana check to have a basic idea of what the spell is and does? I tend to go with 15 + the level of the spell myself. That gives someone trained in arcana with a 16 intelligence (a typical starting wizard) about a fifty percent chance of being basically familiar with any 1st level arcane spell.

So an average intelligence creature has a 30% chance of being familiar with this spell effect and probably knows that most arcane casters can trivially create a small illusion of an object. I tend to give the benefit of the doubt to players if it's a race that's generally less worldly than humans and other PC races. A kobold is probably less likely than the average person who has had a decent chance of seeing this effect by a bard passing through town and entertaining at their local tavern. On the other hand, magical creatures of average or above intelligence are probably as likely to figure out what's going on as any PC who's had a chat around the evening fire with the party wizard about things an enemy caster might do. In other words, they almost certainly know about all the most popular spells--this, illusions in general, polymorph, walls spells, hypnotic pattern, etc.

If you at least pass it off as some other spell effect, you create some doubt even in someone who may be familiar. Honestly though, not much comes to mind for this very limited spell. For something like Silent Image, you can in some cases imitate a conjure animals. Most animals make some noise but a shark underwater was a recent case that I had work automatically for one round until they attacked and their weapons passed through it. The kuo-toa had priests and such and had an idea that some casters could summon animals.

[TheFlyingCleric]

From experience; trying to do this really pisses of the DM and your fellow players. The DM will find some way of putting a spanner in the precariously stacked up arguments-from-RAW, and even if their spanner doesn't technically conform to RAW, they are the DM so it doesn't matter.

Unless you want to get in a half-hour argument about RAW vs RAI, don't try this strat, and just use minor illusion for it's intended purposes; a utility spell designed to be used in clever ways to fool enemies, not one for use when enemies are so close they're breathing down your neck.


Wizard: I cast minor illusion to create a box around myself, hiding me from the cultist who is preparing to strike at me.
DM: The cultist is about to swing, when he observes your hurried gestures, strange from a man about to be run down. When a box suddenly appears where there was none, he pauses only for a moment before attempting to flip the box over with his sword...

[Dalebert]

Exactly. The intention of this spell is not to have combat impact. It can, sure, and I'm more likely to reward players for using it creatively and at lower levels when their options are limited. Dumb goblins and kobolds and such will fall for this crap a lot and you'll get some minor benefit like disadvantage imposed on one attack. But I'm inclined to interpret this extremely strictly to the RAW at the very least because of the tendency to abuse and exploit what is only a cantrip that is already arguably the best and most versatile cantrip, particularly after the lower levels. ANY interaction with it reveals it as an illusion to any observers (and note there's no facing in 5e). One enemy shoots an arrow through your box, it's now transparent to everyone because "physical interaction reveals it to be an illusion" and when someone knows an illusion, it becomes transparent per the RAW. Still, that might save the life of a 1st level caster and is arguably comparable to casting blade ward. No biggy.

Don't push it. If you start trying to blind a creature with a freakin' cantrip by putting a box around its head, I will nitpick every little thing that could go wrong. Is it an enclosed box? Then the creature has already physically interacted with it and it's no good. Is the bottom open and the box is floating in the air? The box appears to be lighter than air and levitating so the creature instinctively reaches up to push it upward as a free action exposing the illusion. Any combat effects are equaled out--it can't see out fully but you've also given it partial concealment. Crap like that. It's just a cantrip. If you try to use a cantrip to imitate a 2nd level spell (blindness), it's going to fail.

[Segev]

Round 1a: Illusionist casts minor image of a 5-ft. cube crate over himself.
Round 1b: Roy the high-Int fighter has seen this trick before, and sighs as he makes his attack through the illusory box with Disadvantage. After that first attack, he can see through the now-revealed illusion.
Round 2a: Illusionist repeats the stunt.
Round 2b: Roy gets increasingly frustrated, because while he's 90% sure it's an illusion, he has to spend an action Investigating it, or swing something through it, so he may as well just attack with Disadvantage again.


Now, if Roy has friends, they all saw his sword pass through the illusory box, and thus can attack without Disadvantage. As can Roy, if he has a second or more attacks.

But each new minor illusion does require its own disproving action(s). Technically, it's up in the air (so down to a DM's ruling) whether an Illusionist can see through his own illusions automatically or not; he knows it's an illusion, but the rules say how it is revealed (successful Investigation, or passing something through it). I'd personally rule the Illusionist gets the (sometimes hefty) advantage of automatically seeing through their own, though, because no matter how much Roy might suspect that box is yet another illusion, he can't know for certain that it wasn't actually a creation by the Illusionist's hidden buddy.

[tieren]

Exactly. The intention of this spell is not to have combat impact.

I disagree it isn't intended to have any impact, I think it is ok, and probably intended for it to have the RAW impact.

That is to say, the caster is then unseen (not with total cover but total obscurement as a previous poster pointed out). To remove the condition generally takes an action at range (shooting it with something or making an investigation check), if enemy doesn't remove the condition it attacks with disadvantage and can't target the original caster with spells that require sight of the target.

If you make it less white room scenario it can get batter. Maybe gnomish Arcane trickster is standing in a 5 foot wide doorway plinking away with a crossbow when he gets the attention of the enemy ranged characters, he then casts minor illusion to put a crate in the door. From the enemy's perspective he may still be in there or he may have used his movement to step out and to the side of the door right after casting, etc...

[Vogonjeltz]

Would the first attack a creature makes against you
a) have complete cover?
b) be disadvantage because they can't see you, similar to invisibility?
c) not be hampered by the illusion at all?

Not cover, that is something physically blocking. It would be obscured (PHB 183) but only for the first attack, because that would reveal the illusion to be an illusion.


Even once something has been revealed to be an illusion, it takes an action and a successful investigation check to see through it.

No:
"Physical interaction with the image reveals it to be an illusion, because things can pass through it."
"If a creature discerns the illusion for what it is, the illusion becomes faint to the creature."

Once interacted with, the illusion is faint, no interaction is required.


EDIT: for all you jerk GMs out there, next time you have the enemy stupidly reach out to touch the illusion that just appeared (thereby proving it to be an illusion), make sure you do the EXACT SAME THING when a creature is summoned, or when a fire suddenly appears, or when the caster shoots out lightning.

Provided the character isn't concerned with the threat of an opportunity attack, there's no reason at all a character couldn't try to walk into the space of an apparently conjure/illusory creature to test the theory. Not that it matters on this subject, minor illusion can't create a creature only sounds/objects.

[SharkForce]

How effective an illusion is depends entirely on how believable it is. Keep in mind that it's a cantrip and one of the best ones, so it's quite possibly the most common spell in the entire world. What kind of DC would you give for the arcana check to have a basic idea of what the spell is and does? I tend to go with 15 + the level of the spell myself. That gives someone trained in arcana with a 16 intelligence (a typical starting wizard) about a fifty percent chance of being basically familiar with any 1st level arcane spell.

So an average intelligence creature has a 30% chance of being familiar with this spell effect and probably knows that most arcane casters can trivially create a small illusion of an object. I tend to give the benefit of the doubt to players if it's a race that's generally less worldly than humans and other PC races. A kobold is probably less likely than the average person who has had a decent chance of seeing this effect by a bard passing through town and entertaining at their local tavern. On the other hand, magical creatures of average or above intelligence are probably as likely to figure out what's going on as any PC who's had a chat around the evening fire with the party wizard about things an enemy caster might do. In other words, they almost certainly know about all the most popular spells--this, illusions in general, polymorph, walls spells, hypnotic pattern, etc.

If you at least pass it off as some other spell effect, you create some doubt even in someone who may be familiar. Honestly though, not much comes to mind for this very limited spell. For something like Silent Image, you can in some cases imitate a conjure animals. Most animals make some noise but a shark underwater was a recent case that I had work automatically for one round until they attacked and their weapons passed through it. The kuo-toa had priests and such and had an idea that some casters could summon animals.

i wouldn't operate under the assumption that the PHB spell list is intended to be totally comprehensive. the PHB doesn't have a spell to create a 5 foot cube box (or a small wall, or a curtain, or whatever), but that doesn't mean we can assume that no such spell exists in the entire world.

[Hrugner]

Round 1a: Illusionist casts minor image of a 5-ft. cube crate over himself.
Round 1b: Roy the high-Int fighter has seen this trick before, and sighs as he makes his attack through the illusory box with Disadvantage. After that first attack, he can see through the now-revealed illusion.
Round 2a: Illusionist repeats the stunt.
Round 2b: Roy gets increasingly frustrated, because while he's 90% sure it's an illusion, he has to spend an action Investigating it, or swing something through it, so he may as well just attack with Disadvantage again.


Now, if Roy has friends, they all saw his sword pass through the illusory box, and thus can attack without Disadvantage. As can Roy, if he has a second or more attacks.

But each new minor illusion does require its own disproving action(s). Technically, it's up in the air (so down to a DM's ruling) whether an Illusionist can see through his own illusions automatically or not; he knows it's an illusion, but the rules say how it is revealed (successful Investigation, or passing something through it). I'd personally rule the Illusionist gets the (sometimes hefty) advantage of automatically seeing through their own, though, because no matter how much Roy might suspect that box is yet another illusion, he can't know for certain that it wasn't actually a creation by the Illusionist's hidden buddy.

Can't he interact with the box as part of his movement? If he knows what's going on he should just be able to step through the box as easily as he could swipe away a bead curtain before attacking someone.

[Segev]

Can't he interact with the box as part of his movement? If he knows what's going on he should just be able to step through the box as easily as he could swipe away a bead curtain before attacking someone.

That would require entering the Illusionist's square. Apply rules for that as appropriate.

[Dalebert]

I disagree it isn't intended to have any impact, I think it is ok, and probably intended for it to have the RAW impact.

It's a little too perfect for small-sized creatures and so much less so if you're medium. I think if they specifically envisioned this as a use of the spell, they might have given a restriction of 5 x 5 x 8 or something similar. I'm just saying, their inspiration for this was probably more cases of people casting it out of combat and out of sight of creatures so that they would suspect it's just a real object.

I want to see more thieves making an illusion of the object they're about to steal and then having a minute of lead-time to get away before the shop-owner notices it missing. :belkar:


That would require entering the Illusionist's square. Apply rules for that as appropriate.

It's at most a free object interaction. He doesn't have to physically enter the square. Only a bit of him does--no more than how much of him enters the square in order to attack, except it's not an attack. It's less effort than pushing open a door that's not stuck which is an example of a free object interaction.

[smcmike]

Can't he interact with the box as part of his movement? If he knows what's going on he should just be able to step through the box as easily as he could swipe away a bead curtain before attacking someone.

Yeah, couldn't he use a free item interaction to touch the box, then attack normally?

[Segev]

Yeah, couldn't he use a free item interaction to touch the box, then attack normally?

I dunno. I'd have to look at what you can do with a "free item interaction." I generally thought those had to be part of preparing/performing another, action-consuming activity.

I would probably lean towards "no," though, on the grounds that "poke the box to see if it's an illusion" probably qualifies as taking that Investigate action. But this really is "ask your DM" territory, I think.

[smcmike]

I dunno. I'd have to look at what you can do with a "free item interaction." I generally thought those had to be part of preparing/performing another, action-consuming activity.

I would probably lean towards "no," though, on the grounds that "poke the box to see if it's an illusion" probably qualifies as taking that Investigate action. But this really is "ask your DM" territory, I think.

There is a list of examples on page 190. They include opening a door and tapping the floor with a ten foot pole (presumably to check for traps). So, yeah, seems pretty clear to me. If not, "fighter uses his move to jump on top of the box."

[SharkForce]

Yeah, couldn't he use a free item interaction to touch the box, then attack normally?

after seeing it once, probably. or, again, if there is some reason to believe this specific creature would be expecting an illusion instead of a spell that conjures a protective barrier or whatever (which should not be the general case; if magic is not common, most creatures won't know about it. if magic *is* common, then illusions should be no more common than transmutations or conjurations generally speaking, and there's no way to definitively conclude that a magically appearing object is more likely to be an illusion than a conjuration or transmutation unless you are looking at the caster's character sheet).

so, after being fooled once into trying to pry open the box instead of attacking, or whatever they would have reasonably done had a box (or whatever) genuinely appeared), you can certainly expect them to interact with the box in some way.

of course, you can also potentially exploit that, too :)

[Dalebert]

so, after being fooled once into trying to pry open the box instead of attacking, or whatever they would have reasonably done had a box (or whatever) genuinely appeared), you can certainly expect them to interact with the box in some way.

As I said earlier, I'd be inclined to give the PC the benefit of the doubt at lower levels with really dumb and not very worldly creatures like kobolds and goblins, but even most humans will have some idea that illusions are a thing with magic. They've probably seen a bard use Minor Illusion in a tavern for a show or at least heard about them. Magic isn't that uncommon as you start facing higher level creatures.

Given that the cost to just check is nearly nothing (a free object interaction to reach out and touch) then there's really no reason for a reasonably intelligent creature not to check before wasting meaningful actions. (see my signature) If you realize it's actually solid, just move and attack something else. Investigation is really only needed if you're not close enough for that or if it's a more powerful effect like Phantasmal Force where just touching it won't reveal the illusion because it will feel real.

But let's face it. A gnome isn't going to use Phantasmal Force to make a box around itself. It's going to use that to make a cage of red-hot bars around YOU.

[SharkForce]

who says the same people that have seen minor illusion in taverns have never seen a conjurer use their subclass ability? or that they've never seen someone create a wall of dirt or ice using cantrips? who says they've never seen someone cast all manner of other spells?

you may as well argue that people will immediately attack a bard if the bard ever insults someone because vicious mockery is a cantrip that lets you insult people to death. why is everyone immediately perfectly familiar with the one spell only, and how do they know that the person they're dealing with knows minor illusion but doesn't know a spell called "conjure minor wall" or something like that?

i mean, seriously, does the spellcaster yell "minor illusion" at the top of their lungs as if they're in a comic book or something? does the wall have a sign on it that says "this wall is fake"?

if you literally just pulled that trick on them 6 seconds ago, then sure they'll think you're doing the same thing. if they're in a city that is ruled by a council of illusionists that drive out other spellcasters, no problem. if you're in a random ruined city in the middle of nowhere, populated by a variety of beasts that can do all kinds of different things, how do they know that this time when you cast a spell that makes something it's probably an illusion but they don't "know" the same thing when you cast magic missile or flaming sphere? i mean, they've probably seen people hide in various places before. as soon as the rogue hides, do they know the location the rogue is hiding in because they're familiar with hiding? they've seen people fight with swords before. does that mean any time the fighter tries to use a sword, they know exactly what the fighter is going to do and can parry consistently because of it?

maybe it's an illusion. maybe it isn't. the first time they see you do something, they have no way of knowing whether you're using a typical illusion spell or a spell that creates the thing you're seeing for real. after being fooled once (recently), sure you'll be cautious. maybe even if you just walked out of an illusion show. but most of the time, no, you're going to expect that what you see is what is there.

[Dalebert]

who says the same people that have seen minor illusion in taverns have never seen a conjurer use their subclass ability? or that they've never seen someone create a wall of dirt or ice using cantrips? who says they've never seen someone cast all manner of other spells?

You're going off on a bunch of tangents that aren't particularly relevant. Minor Illusion is arguably the most commonly known spell in the world because it's super useful and versatile and because it's just a cantrip and it's on a lot of spell lists. But even that isn't as relevant as the fact that people will know that illusions are a thing in general whether they know of any specific spell.

In many cases it boils down to two things.
1) Might it be a simple illusion? (like visual only or audiovisual only, i.e. the most common ones)
2) How hard is it to confirm?

If something just appeared and you're aware that illusory magic is a thing, and if you can trivially test it by just reaching out and touching it with a free action, then why wouldn't you? It's the most rational response so if the creature is reasonably intelligent, it generally would.

[JackPhoenix]

Touching a suspicious "wall" doesn't cost you anything (object interaction usualy isn't that needed)... there's no reason not to do that.

People know illusions exists, they likely know mages capable of creating real things exist too, even if they personally never met them. The former is a much more common, but there's still a chance its one of the later. It's about risks:

If you're an archer and the enemy spellcaster suddenly created a wall to hide behind, why not shoot at it? Best case scenario, it's an illusion and you hit the foe hiding behind it. Worst case scenario, it's a solid wall, you've wasted an arrow and an attack... no big deal, if there wasn't any other target or an opportunity to get a better line of sight.

Now, if you were chasing a spellcaster through a corridor and he created a wall of flames (impossible to do with Minor Illusion, but let's say it was Silent Image or better to simulate the heat) to block your path... there's now substantial risk in presuming it's an illusion: best case scenario, it's an illusion and you can go right through. Worst case scenario? It's real and you'll get horribly burned if you try that. Most people wouldn't risk it, though they may try to stick something into the flame and see if it really burns, thus giving the caster time to escape. Still, enraged orc may ignore the flames and run straight through, his bloodlust and hatred towards the caster being stronger than his fear of burns.

It's not bad or antagonistic GMing if the NPCs do that... it's logical, in character action. After all, the player characters would do the same in the same situation. (happened in my game with the second example: the characters were reasonably sure it's just an illusion, but they still weren't willing to risk their own skin, until the paladin decided that being burned is acceptable risk compared to the enemy escaping).

[tieren]

Yeah, couldn't he use a free item interaction to touch the box, then attack normally?

I am ok with a quick interaction to check an illusion in melee range, but I think it becomes a question of how trained the subject is to do that as to how much of their action it should take.

Another adventurer or an experienced fighter, a free interaction seems appropriate. For someone that might need to figure out what is going on I wouldn't treat it as natural as opening a door (which is instinctual and not something which requires much decision making).

Picture a combatant that is suddenly faced with a stone wall in front of them. I do think they would interact with it, but for some people I think that might include dropping their guard or lowering their weapon for a second while they do that, there should be a mechanical cost for that, I believe losing the action for an investigate check to be the simplest. For a more trained opponent maybe just losing a bonus action, and for a seasoned adventurer just doing the free object interaction.

[Blue Lantern]

Touching a suspicious "wall" doesn't cost you anything (object interaction usualy isn't that needed)... there's no reason not to do that.

People know illusions exists, they likely know mages capable of creating real things exist too, even if they personally never met them. The former is a much more common, but there's still a chance its one of the later. It's about risks:

If you're an archer and the enemy spellcaster suddenly created a wall to hide behind, why not shoot at it? Best case scenario, it's an illusion and you hit the foe hiding behind it. Worst case scenario, it's a solid wall, you've wasted an arrow and an attack... no big deal, if there wasn't any other target or an opportunity to get a better line of sight.

Now, if you were chasing a spellcaster through a corridor and he created a wall of flames (impossible to do with Minor Illusion, but let's say it was Silent Image or better to simulate the heat) to block your path... there's now substantial risk in presuming it's an illusion: best case scenario, it's an illusion and you can go right through. Worst case scenario? It's real and you'll get horribly burned if you try that. Most people wouldn't risk it, though they may try to stick something into the flame and see if it really burns, thus giving the caster time to escape. Still, enraged orc may ignore the flames and run straight through, his bloodlust and hatred towards the caster being stronger than his fear of burns.

It's not bad or antagonistic GMing if the NPCs do that... it's logical, in character action. After all, the player characters would do the same in the same situation. (happened in my game with the second example: the characters were reasonably sure it's just an illusion, but they still weren't willing to risk their own skin, until the paladin decided that being burned is acceptable risk compared to the enemy escaping).

I completely agree with what you said.

[Dalebert]

Now, if you were chasing a spellcaster through a corridor and he created a wall of flames (impossible to do with Minor Illusion, but let's say it was Silent Image or better to simulate the heat) to block your path...

Brilliant points and I want to expound on this one. A wall of stone has no consequence to touching it, real or not. You may as well burn a free action that you probably weren't going to use anyway, assuming you're right there or were going that way anyway like to chase someone down a corridor. A wall of fire has obvious repercussions. But as you said, unless you use a more powerful illusion like Major Image, the lack of heat would quickly give it away. People would get right up to until they felt heat, i.e. never, and then go right through. Major Image won't burn you, but it will feel hot at least and thus be more convincing. The point here is the weaker lower-level illusions are going to be more limited in their use. They're not intended to be very useful in many combat situations and you just have to use them more cleverly to get more bang for your buck. A wall of stone will be much more effective if you get around a corner and go through a door, then use the wall to hide the door. Just throwing up a wall of stone illusion with a piddling Silent Image right in front of enemies is going to be borderline useless in most situations. And my point is that's intended and nothing to complain about. It's only a 1st level spell.

[Hrugner]

I am ok with a quick interaction to check an illusion in melee range, but I think it becomes a question of how trained the subject is to do that as to how much of their action it should take.

Another adventurer or an experienced fighter, a free interaction seems appropriate. For someone that might need to figure out what is going on I wouldn't treat it as natural as opening a door (which is instinctual and not something which requires much decision making).

Picture a combatant that is suddenly faced with a stone wall in front of them. I do think they would interact with it, but for some people I think that might include dropping their guard or lowering their weapon for a second while they do that, there should be a mechanical cost for that, I believe losing the action for an investigate check to be the simplest. For a more trained opponent maybe just losing a bonus action, and for a seasoned adventurer just doing the free object interaction.

This sounds about right. Would a passive investigation check against spell DC make sense as a determiner for whether or not the person can free action the illusion away?

[Dalebert]

This sounds about right. Would a passive investigation check against spell DC make sense as a determiner for whether or not the person can free action the illusion away?

No. If you touch it, it's automatic--no check needed. If you're too far to touch it, then you have to spend an action to investigate and also succeed against the illusion's DC.