But this is the problem. Other than "Looking for danger" how else do you look for danger? Get too specific, and you are just making an investigation check, not a perception check.
An investigation check is most definitely not just a more specific perception check at my table. I mean, for one thing I call for the ability and let the player determine if one of their proficiencies applies, so it’s really just a wisdom check, to which you can decide if you think your perception or investigation proficiency (or some other proficiency or no proficiency at all) applies. But, by default the investigation skill is a specific application of the intelligence ability. According to the PHB, “an Intelligence check comes into play when you need to draw on logic, education, memory, or deductive reasoning” and proficiency in the investigation skill applies “When you look around for clues and make deductions based on those clues.” In contrast, Perception is a specific application of wisdom. The PHB says “A Wisdom check might reflect an effort to read body language, understand someone's feelings, notice things about the environment, or care for an injured person” and perception proficiency applies when you try to “spot, hear, or otherwise detect the presence of something.” In other words, Perception is for finding the clues, investigation is for deducing their meaning.
Or you risk asking the wrong question, and getting false positives.
P: "I step partways forward, paying special attention to the wardrobe."
DM: "You don't see anything near or in the wardrobe"
P: "Okay, I finish stepping into the room and head towards the chest"
DM: "The assassin's blade stabs you in the back, he was behind the door. You didn't look there."
I don’t see anything unreasonable in that exchange.
Well… Except I guess for the part where the assassin successfully stabs the PC in the back without initiative being rolled or an attack roll being made. But I don’t think that was what you were meaning to use the example to illustrate.
OKay, but let me give you a counter example here.
You have finished describing an idol. The cleric, who was an acolyte in their backstory, holds up the die and asks "Religion?" Do you really need them to say "I want to think back to my training as an acolyte of the Sun Lord to see if I have heard anything about this idol before."?
Yes. From them holding up the die and saying “religion?” I can reasonably surmise that they want to know something about the idol, and that they think their proficiency in religion would be applicable if a roll is required. But they have not conveyed any information about what they want to know, nor have they provided me with enough information to determine whether the attempt to learn that information could succeed or not.
I mean... it is very obvious they are asking about the idol, and that they are asking if they have run across it in their studies, whether as an acolyte or later.
Asking what about the idol though? Are they asking if they’ve run across it in their studies? If so, they could simply say that, instead of relying on me to correctly guess that instead of making a different assumption. Maybe I think they’re asking if it depicts a god they’re familiar with. Maybe I think they’re asking if it displays any signs of having been desecrated. Point is, I can’t read their mind, nor am I interested in trying to guess what they meant.
Because the Religion skill proficiency doesn't actually require or state anything about HOW they gained the proficiency. Did they study religious texts? Were they trained by a bard? Did they just grow up and speak to a bunch of old priests? The skill itself doesn't require anything about how you got it, so why am I required to specify why I might know something about something I've never seen before.
I’m not asking where they acquired their proficiency, I’m asking where they imagine their character might have learned the information they want to know about the idol. Doing so gives me something to assess to determine if the action can succeed or fail and what check to call for if both are possible, and it reveals an interesting detail about the character’s backstory.
But why? Knowledge and perception and insight are completely passive. And by that I mean, I don't make "knowledge" checks in real life by doing any specific action.
You don’t make checks in real life at all. They’re a gameplay concession, used to resolve uncertainty.
I just know what I know. If I need to do an action, like pulling out a book or searching my phone, then I'm doing research. And generally, if I have access to research materials, the DM calls for an investigation check to go through them, not a knowledge check.
Again, I just call for an Intelligence check in either case and let the player determine if one of their proficiencies applies.
I don't walk into a room and perceive things by taking any specific action. In fact, most of the actions I've seen laid out for "perception" are either altering the environment (throwing in a torch to illuminate the room) or are investigation. Neither of which is meant by a perception check.
When you walk into a room, I describe anything you can perceive that isn’t hidden, and use your passive perception to determine if you notice anything that is hidden. That’s how the rules indicate that such general “looking around” should be resolved. If for some reason you suspect there is still something hidden that you missed with your passive perception, you have to
do something if you want to find it, and you need to tell me what it is you do. Is what you do altering the environment? Will an Intelligence (Investigation) check be needed to resolve it? I can’t know until you tell me what it is you do.
If I'm listening to someone talk, I don't reach out and grab their face to see if they are lying, or write down their words and reread them to see if they are lying, I'm already doing the pertinent action. Listening and looking.
Great, and for that you get to apply your passive ability (skill).
By doing what? What action can they possibly take if you have clued them off by saying the NPC is "unusually sweaty" or something. You've either given them enough clues they are just looking to confirm via insight, or they are looking for insight to give them the truth.
If the players think that me describing the NPC as unusually sweaty is enough to confirm that the NPC is lying, they are free to proceed according to that assumption. I generally advise that it’s smart play to take steps to confirm your assumptions, but that’s the players’ decision to make.
But most players want to roll to find out if they missed any clues that the person was lying.
They didn’t miss any clues. I described what they perceived, as it is the DM’s role to do. They are free to decide what to do with that information. Again, I advise that it’s smart play to try and confirm their assumptions through action, but it’s up to them.
And it is really hard to look at me acting like the NPC and determine if they are lying, because of course I'm lying, I'm trying to act and I'm a bad actor.
Well, I’m a pretty damn good actor, but even so, I don’t want to rely solely on my acting ability to convey clues about a character’s emotional state. For one thing, I’ve played with people who have difficulty reading social cues and I don’t want them to be at a disadvantage because they can’t tell that I’m acting like someone who’s trying to hide something. No, I’m going to
describe whatever the cue is, narratively, rather than relying on the players’ real life ability to read my behavior.