Question about a rule for illusions

[Throne12]

Doesnt it take a action to roll a investigation to figure out if the illusion is real?

[Sorinth]

It depends.

Generally illusions allow you to spend an action to disbelieve but also have the caveat that physical interaction reveals them to be illusions. So someone might do something that isn't an action can reveal it to be an illusion. But there are exception like to that like Phantasmal Force which tricks the mind into still believe the illusion.

[prabe]

I believe the rule is that it takes an action, yes, but it's not unreasonable for it to happen in the course of other things. Maybe there's an illusory object someone tries to interact with, or maybe an Inquisitive Rogue could do it as a bonus action.

[Tanarii]

It's specific to the illusion. But several of them allow an action to make an Intelligence Investigation check, and have it go faint if you succeed in the check.

They also tend to have physical interaction reveal the illusion. But given the way they are formatted, IMO that doesn't make it go faint. It just lets either the person making the interaction or anyone that sees the interaction (unclear which from RAW) know it's an illusion, so they can then either behave accordingly or take an action to make it go faint so they can see through it. I know that's an edge interpretation, but I feel it fits the way RAW is written better and makes running illusions work better as a DM.

[Man_Over_Game]

I believe the lead developer is of the mindset that basically anything that spent an Action to do something and may lead to a clue that reveals the illusion is enough to warrant the Investigation Check.

Swung your sword and missed? Investigation Check.
Cast a spell and something about the lighting seemed funny? Investigation Check.

The thing is, Illusions appear real until they need to interact with something (like lighting and the such), and circumstances often change whenever people are using Actions to do stuff (like fighting, talking, walking around, etc.) and so the Illusion can be caught in almost any moment where more than NOTHING is happening and the observer is paying enough attention to notice.

And I agree with that. I don't need you to spend your Action on an Investigation Check, I just need you to spend your Action and do something that might lead to one. What this means is that people in passing, or people in a hurry, or most people who aren't observant won't have a reason to roll an Investigation Check. A merchant who's watching your eyes to tell if you're bluffing on your haggle is not the person you want to use Disguise Self on, since he's already paying close attention to your appearance and will get many attempts to see through your disguise.