When is Passive Investigation used?

[Man_Over_Game]

I'm asking the same question on RPG Stack Exchange (https://rpg.stackexchange.com/questions/131239/when-is-passive-investigation-used#131240), for those who are interested.

In the Sunless Citadel module, from The Tales From the Yawning Portal, passive perception is mentioned about half a dozen times, but passive investigation isn't mentioned once.

However, both passives are increased by +5 by the Observant feat, and both are included when it comes to the Inquisitive Rogue subclass in XGtE. In most mechanical and class mentions of passive perception vs. passive investigation, it seems they are supposed to be equals.

Passive perception is used as the defensive stat for enemies attempting to stealth, but no such claim is made for passive investigation.

So when is passive investigation used, beyond being an automatic permanent minimum roll for an unmade active investigation check?

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Personally, I like to use them as a reactionary stat, for things that you can miss (somebody passing by, a drop of water falling from the ceiling, knowing the intended target of a shot fired from an assassin). This is not, however, an official use of the passive skills. I'm interested in regular uses of the passive skills for the sake of this question.

[mephnick]

I find it really hard to think of a circumstance where you'd be investigating things over and over enough to require a passive check, but I guess it has its uses as a secret check for secret doors?

[Man_Over_Game]

Does that imply that Passive Perception shouldn't be used for the same thing?

[mephnick]

Does that imply that Passive Perception shouldn't be used for the same thing?

It's pretty arbitrary, yeah. For a secret door I'd allow PP to detect an unnatural airflow that hints at a passage, but still require Investigation to reveal it and show how to open it, much like how a trap works.

[Tanarii]

The big ones are:

Figuring out via deduction the location of hidden things, or by finding/realizing clues to their location. This includes trap triggers, the trap mechanism, secret compartments, and secret doors.

Figuring out how something works, so you can do something with it. Includes how traps work, so you can disable them. How secret compartments and secrets doors are opened.

Deducing / realizing something is clue to something. This is a more generalized version.

In some cases, a passive perception check by someone (not necessarily the investigator) may be necessary first to notice these things, as opposed to just flat overlooking them. In other cases, merely looking in the right place will automatically cause you to see the thing.

Basically, the question for resolution for Investigation is if you deduce or realize the clue / location / way something works, either after it's been noticed or when its in plain view.

Edit: passive investigation follows all the normal rules for passive check use. You use it when:
- a PC is doing something over and over again and you don't want to roll constantly, such as investigating passages for traps as they go.
- the thing is secret (from the player) and a rolled check would give it away.

In this regard, I personally consider passive investigation to automatically be used almost any time passive perception is. But IMx it's also far more often used non-passive than perception, as a one time check on non-secret things they know are there.

[Demonslayer666]

When someone is looking at a crime scene and doesn't say they investigate.

[Man_Over_Game]

Just a quick reminder, guys.



So when is passive investigation used, beyond being an automatic permanent minimum roll for an unmade active investigation check?

[Ganymede]

The rules suggest that you can use a Passive check whenever you don't want a die to be rolled.

I apply this by allowing my players to essentially "take 10" in low pressure situations. I also generally have NPCs use passive checks when they do something contested by a PC's check as a way of setting a DC without a roll.

[Tanarii]

So when is passive investigation used, beyond being an automatic permanent minimum roll for an unmade active investigation check?
What exactly are you trying to say here? Are you pointing out that passive checks aren't passive in the part of the Pc, but rather the player? Because while it may sometimes be an effective minimum, if a rolled check is made after a passive check, it's never an actual minimum.

Just like Passive perception, it follows the rules for when it is used in place of a rolled check:
- when something is being done over and over again and you don't want to keep rolling.
- when the fact a check is being made needs to be kept secrets from the player.

If those aren't the case, it should either be some other kind of check.

Personally I generally use a priority tree for deciding what kind of check to use:
1) is it automatically successful or automatically a failure? No check.
2) is it automatically successful with time, and the player has the time? Give them the option to use time (or default to that if time doesn't matter in your campaign), and no check.
3) can the player even know a check is being made? Do I want to avoid rolling checks over and over again? Passive score.
4) is the group working together, with some people making the situation worse and others better? Group check.
5) is it actively opposed by someone? Opposed check.
6) set DC, rolled check.

[Man_Over_Game]

What exactly are you trying to say here? Are you pointing out that passive checks aren't passive in the part of the Pc, but rather the player?

I'm not sure what you mean by the second question, but my goal is to find why someone would need to deduce things without actively trying to do so.

However, the answers that stand out the most seem to be "To keep information hidden from the player", which is feasible and workable.


A key component to this question hinges on the mechanical aspects of passive perception, as it's how stealth is defended against on a regular basis, but there's no regular mechanical use for passive investigation.

I guess a better way of asking would be, how would a DM use passive investigation in a way that it becomes just as relevant as passive perception? Or how is passive investigation better than passive perception?

[MaxWilson]

I find it really hard to think of a circumstance where you'd be investigating things over and over enough to require a passive check, but I guess it has its uses as a secret check for secret doors?

I think it's appropriate for highly-observant, PI-type characters. In this scene I think Harry Dresden shows off his passive investigation skills:


“Thomas,” I whispered. “Please, man. This isn’t what you want to be. I know you, man. I’ve seen you.”

“You’ve seen who I wanted to be,” he said. “Who I thought I was.” He shook his head and looked around at the people around us. “Play a game with me.”

“What game?”

He nodded toward a pair of young women walking by holding ice-cream cones. “What do you see when you look at them? Your first thought.”

I blinked. I looked. “Uh. Blonde and brunette, too young for me, not bad to look at. I bet the blonde paid too much for those shoes.”

He nodded and pointed at an old couple sitting on a bench. “Them?”

“They’re fighting with each other over something and enjoying it. They’ve been together so long, it’s comfortable for them. Later, they’ll hold hands and laugh over the fight.”

He pursed his lips, and pointed at a mother chivvying a trio of small children of various sizes along the zoo. “Them?”

“She’s got an expensive ring, but she’s here at the zoo alone. Her kids all have matching outfits. Her husband works a lot, and she doesn’t look as good as she used to—look how the shoes are
biting into her feet. She’s worried that she’s a trophy wife, or maybe an ex-wife in progress. She’s about to start crying.”

“Uh-huh,” he said. “Can I give you my first thoughts?”

I nodded, frowning at him.

Thomas pointed a finger at the young women. “Food.” He pointed a finger at the old couple. “Food.” He pointed a finger at the mother and her children. “Food.”

I just stared at him.

Harry doesn't have to spend time (i.e. player attention) reasoning about these people. Certain things are just obvious to him. If you wanted to simulate this in-game, I would say, "mostly don't bother," but if you did I'd say "prepare an index card with additional random facts about certain NPCs, write the passive investigation DC on the back, and toss it on the table when the NPC comes onscreen. Any player whose PC has a passive Investigation that beats that DC gets to flip over the card and read it if she wants to." If I were going to do this in a game I would mostly focus on giving significant info about NPCs/clues that matter to the outcome of the game, but I'd also prepare a few boilerplate cards that I can throw down for unimportant NPCs and locations to make it less blindingly obvious that tossing down a card is a "plot" clue. I'd probably try to aim for a 50/50% ratio on significant vs. insignificant deductions.

Basically you're trying to make it clear to the player that he notices a LOT of random stuff about other people, some of it significant, but you're trying to do so in a way that doesn't bog down the game.

[Man_Over_Game]

I think it's appropriate for highly-observant, PI-type characters. In this scene I think Harry Dresden shows off his passive investigation skills:



Harry doesn't have to spend time (i.e. player attention) reasoning about these people. Certain things are just obvious to him. If you wanted to simulate this in-game, I would say, "mostly don't bother," but if you did I'd say "prepare an index card with additional random facts about certain NPCs, write the passive investigation DC on the back, and toss it on the table when the NPC comes onscreen. Any player whose PC has a passive Investigation that beats that DC gets to flip over the card and read it if she wants to." If I were going to do this in a game I would mostly focus on giving significant info about NPCs/clues that matter to the outcome of the game, but I'd also prepare a few boilerplate cards that I can throw down for unimportant NPCs and locations to make it less blindingly obvious that tossing down a card is a "plot" clue. I'd probably try to aim for a 50/50% ratio on significant vs. insignificant deductions.

Basically you're trying to make it clear to the player that he notices a LOT of random stuff about other people, some of it significant, but you're trying to do so in a way that doesn't bog down the game.


Props on the Dresden quote. Really looking forward to Peace Talks, myself.

A fair point. I guess both passives come down to the narrative each player receives when they get descriptions. The system seems lopsided since the perception version works against stealth, which is less of a narrative aspect but a more combat/mechanical situation that no other passive skill gets, but perhaps that's just the hand we're dealt.

In a perfect game, each skill would be just as important as every other skill, but when would Medicine, with Expertise and a level 20 character be more powerful than a level 5 cleric?

[Tanarii]

I'm not sure what you mean by the second question, but my goal is to find why someone would need to deduce things without actively trying to do so.Oh i had it backwards then. Your question is based on misunderstanding what the passive check rules are for.

They aren't for when a character isn't actively trying to do something.

They are for when a player isn't actively rolling a die. Either because they can't know a check is happening, or because it would require rolling over and over again.

Its passive on the part of the player, not the character. The character can be, even usually is, doing something active.

[MaxWilson]

Props on the Dresden quote. Really looking forward to Peace Talks, myself.

A fair point. I guess both passives come down to the narrative each player receives when they get descriptions. The system seems lopsided since the perception version works against stealth, which is less of a narrative aspect but a more combat/mechanical situation that no other passive skill gets, but perhaps that's just the hand we're dealt.

In a perfect game, each skill would be just as important as every other skill, but when would Medicine, with Expertise and a level 20 character be more powerful than a level 5 cleric?

(1) Combat uses for Investigation: I suppose you could let it bust illusions without requiring an action to investigate, if the illusion DC is lower than passive Investigation. "You open the door and see three Death Knights and a fire-breathing Adult Red Dragon. The dragon is obviously an illusion." That's a useful bit of combat info to have.

(2) I like to allow Medicine for autopsies. Spend an hour after combat dissecting a monster's corpse and make a successful Medicine check and I'll tell you its Str/Dex/Con/Int/Wis/Cha/AC/HP/attacks/actions. (Yeah, technically it doesn't make sense to deduce Int/Wis/Cha from an autopsy but I haven't worried about that much.) Sometimes but not always I apply a random noise component to the stats before handing them out, based on how good the Medicine check was. (E.g. if "barely succeeding" when roll total exactly equals target DC, stats may be off by 50%.) Other times I will just pull out the MM and show the player the stat block--it depends on how much effort I'm prepared to invest in prepping the creature. (And sometimes I don't even ask for a Medicine roll as long as you're proficient in Medicine. I'm not anywhere near 100% consistent on this issue.)

For magical abilities and highly-magical creatures it would be an Arcana check instead.

Having information about what you're up against is powerful along a completely different dimension than a 5th level cleric's spells.

[Man_Over_Game]

(1) Combat uses for Investigation: I suppose you could let it bust illusions without requiring an action to investigate, if the illusion DC is lower than passive Investigation. "You open the door and see three Death Knights and a fire-breathing Adult Red Dragon. The dragon is obviously an illusion." That's a useful bit of combat info to have.

(2) I like to allow Medicine for autopsies. Spend an hour after combat dissecting a monster's corpse and make a successful Medicine check and I'll tell you its Str/Dex/Con/Int/Wis/Cha/AC/HP/attacks/actions. (Yeah, technically it doesn't make sense to deduce Int/Wis/Cha from an autopsy but I haven't worried about that much.) Sometimes but not always I apply a random noise component to the stats before handing them out, based on how good the Medicine check was. (E.g. if "barely succeeding" when roll total exactly equals target DC, stats may be off by 50%.) Other times I will just pull out the MM and show the player the stat block--it depends on how much effort I'm prepared to invest in prepping the creature. (And sometimes I don't even ask for a Medicine roll as long as you're proficient in Medicine. I'm not anywhere near 100% consistent on this issue.)

For magical abilities and highly-magical creatures it would be an Arcana check instead.

Having information about what you're up against is powerful along a completely different dimension than a 5th level cleric's spells.

That's a solid idea on #1 to be used consistently.

Stealth vs. Passive Perception

Illusion vs. Passive Investigation

Deception vs. Passive Insight.

Perhaps passive investigation never gets used because illusions aren't as common as stealth or lies. This does give me a few ideas, though, to make some illusion-based encounters.

Like an Indiana Jones-esc boulder that chases the team down a hallway. Down the hallway is a spike pit, illusioned to appear like normal floor. The boulder is also an illusion. The idea is that two illusions + 1 pit is a lot easier and more sustainable to use than lifting one giant boulder that's going to destroy your temple every time a large rat sets off your trap. The boulder makes sound, but it doesn't damage the walls or shake the floor, so it is seen through an investigation check, or if someone is stupid enough to try and stop it. A cool idea, but I don't know if players would be cool with being tricked into jumping into a pit of spikes.

[Arcangel4774]

Passive perception is often used when you want a check but a dice roll would give something away. Investigation could work much the same.

Dm: you walk into a dark and damp cave with a smooth floor. Theres glowing mushrooms giving you the the slightest bit of light. As you walk you notice that it gets cooler as the farther in you go.

Passive perception could add " you hear the quiet sound of waves lapping at the shore in the distance"

Passive investigation could add "based on the temperature change and dampness you suspect theres some body of water nearby"

[MaxWilson]

Like an Indiana Jones-esc boulder that chases the team down a hallway. Down the hallway is a spike pit, illusioned to appear like normal floor. The boulder is also an illusion. The idea is that two illusions is a lot easier and more sustainable to use than one giant boulder that's going to destroy your temple every time a large rat sets off your trap. The boulder makes sound, but it doesn't damage the walls or shake the floor, so it is seen through an investigation check, or if someone is stupid enough to try and stop it. A cool idea, but I don't know if players would be cool with being tricked into jumping into a pit of spikes.

*Yoink!*

Consider this idea, and multiple variations on it, stolen. :-)