Who decides when you roll or take a passive perception check (especially for traps)?

Question

I'm about to start playing DnD 5e with my family. I'm the DM and I'm totally new. I've read the PHB and have decided to do Grammy's Country Apple Pie as the first campaign.

In the campaign, the players are searching for two halves of a recipe. They are told that one half is in the office. In the section for the office the campaign guide says:

One of the desk drawers is trapped, but the mechanism can be spotted with a DC 13 Perception check.

But I'm slightly confused as to who instigates the check. Does the player say "I look around near the desk", and then I say to roll a perception check? Or once they're near the desk, do I say "roll a perception check", or do I use a passive check so as to not make the players aware that there could be a trap.

Is the idea that if they say nothing before opening the drawer, the trap gets them? Or do I force them to make a check to decide if they notice the trap or not?

What is typically done in this case when there are traps?

Answer 1

The basic game loop for D&D is:

1. The GM describes the world.
2. The player describes what they want to do, given that state.
3. The GM decides whether what they want to do is possible and how difficult it is. If necessary, a die is rolled to determine success ("Rule #2: Only Roll When There is Chance of Success, A Chance of Failure, and A Risk or Cost of Failure" -- AngryGM).

They might want to do things like "kill the goblin", "investigate the desk", or even "loot the dungeon". So, some things might take several rounds to accomplish or several checks.

The trick in this particular scenario is that the Wisdom (Perception) check likely happens during step 1, not step 3; step 3 is where an Intelligence (Investigation) check is likely rolled.

The rule of thumb that has worked for me is succinctly put by this random Reddit comment (there are several other nuggets in the thread):

Perception is noticing there is a book case, investigation is looking through the books.

Perception is discovering a body in the corner, investigation is examining it.

Perception is looking for an escape route, investigation is looking for the switch that opens the passageway.

Basically, if they're looking around in a broad space it's perception & if they're carefully examining or combing through something it's investigation.

So:

1. Describe the world. If the character's passive perception is 13 or higher, they notice that something is off about the drawer. If the player talks about "carefully opening the drawer" or "searching the desk for traps", they should get an active Wisdom (Perception) check to see if they notice something being "off", too. In either case: this may happen as they start to open the drawer, but definitely happens before they've opened it enough for the trap to trigger. That puts us back into step 1: describe the world. The description is now (eg.) "As you start to open the drawer, you feel an odd catch".
2. The player describes what they want to do. This could be "I pull the drawer open anyway" or "I stop and check to see what's catching" (or one of ten million variations, of course).
3. The GM adjudicates.

There are a few options for step 3:

• If the character didn't notice anything amiss in step 1, they succeed in opening the drawer; no check is required. Unfortunately, the trap goes off. Note that this option doesn't actually have a second round of the basic loop.
• If the character noticed something amiss in step 1 but carried on opening the drawer anyway, it's the same result: the drawer opens and the trap goes off. There is a second round of the basic loop here, but the end result is still "the drawer is open and the trap has gone off".
• If the character noticed something in step 1 and chose to investigate, the GM likely calls for an Intelligence (Investigation) check to see if the character can determine how the trap mechanism works well enough to attempt a bypass; if they succeed, go back to step 1 (eg., "You see a small latch on a spring; opening the drawer will cause the latch to rise, setting off a trap of some sort") and see how the player wants their character to respond (likely using thieves' tools to bypass the trap, but there could be other perfectly reasonable options, too).

Answer 2

TL;DR Check with your DM

You never "take" a passive Perception check. If your DM is using the guidelines from the basic rules, whenever you have a chance to detect a hidden object, like a secret door or loose floor tiles over a pressure plate, or a hiding enemy, the DM should be comparing the PC's passive Perception to the search DC (Stealth check for creatures) of anything you may notice. If you are actively searching, the DM should be comparing the results of your roll. A lot of people state the passive check is a floor for the active check, but in reality, you should never be making an active check for something you already have discovered, so it would never actually come into play, unless your DM forgot to check.

Hidden objects and hiding enemies, from chapter 7, Using Ability Scores

A passive check is a special kind of ability check that doesn't involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

Traps, from Chapter 15, Running the Game

A trap's description specifies the checks and DCs needed to detect it, disable it, or both. A character actively looking for a trap can attempt a Wisdom (Perception) check against the trap's DC. You can also compare the DC to detect the trap with each character's passive Wisdom (Perception) score to determine whether anyone in the party notices the trap in passing.

Surprise, from Chapter 9, Combat

The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn't notice a threat is surprised at the start of the encounter.

It's important to note, there's a lot of the DM can language as well as, from the DMG's introduction,

The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.

So, your milage may vary, that is, in general, check with your DM.

Since you are the DM

In your case, you are the DM, so you get to decide. What works for me is, I always compare passive Perception before I describe the scene, so that information is included when I lay out the description. If a player wants to then actively search, then I ask for the appropriate roll, or simply offer the information, if the player's choice is self evident.

For example, if there's a key in the dresser drawer under some clothes and the player says

I search the dresser

and I say,

It's filled with folded shirts and garments

and the player says,

I pick them up and look under them

I say,

You find a key.

No rolls, just role play.

Answer 3

Passive checks are a DM tool. They largely exist so that the DM can use the PC's competencies to decide something without even telling the players that a check is being made, either to avoid revealing information even on a failed check, or just to speed things along.

We've all been in games where the DM calls "everyone make a perception check", looks at the results, and then says "nobody notices anything unusual"; even with the best of intentions not to metagame it's still hard for the players not to go on alert after that!

In particular, passive perception is supposed to represent what the PCs notice passively. It shouldn't require any specific action or declaration from the player that their PC is looking around, instead it "just happens" when they're in the right circumstances. Passive perception tells you whether they get information when they're just in the presence of something they could notice, but is hard to notice. It gives you, the DM, a way to decide whether or not to include the hard-to-notice feature in your description (that isn't just making an arbitrary decision).

Passive perception still requires the DM to make a judgement call about exactly when the PCs could notice something; what are the "right circumstances"?. You might decide that just entering the room with the desk gives them the chance to spot the trapped drawer; in many ways this is simplest because you almost certainly know in advance which PCs passive perception is high enough (the only reason you couldn't is if there are temporary effects altering their perception), and just factor that in to the way you describe the scene in the first place. Or you might decide that it's just too small/subtle to be noticed at all from across the room, but anyone who goes over to the desk might spot it, and save the description for that point. Or you might decide it's unnoticeable unless you're actually looking at the specific drawer, which doesn't happen unless a PC is actually opening the drawer or otherwise describes their character looking at the drawers. (But note in this last case it's still not the act of looking at the drawers that triggers the passive check, it's that that act puts them in the right circumstances for noticing to be possible)

So it's ultimately up to the DM exactly when the passive check happens. You can decide the possibility to notice something is contingent on the players doing something more than just "existing in a scene with it". But you probably shouldn't let a player trigger a trap without having at least checked their passive perception at some point; even if a player says "I open the drawers of the desk" as soon as they walk in the room, I wouldn't take that as saying that they ignore any signs their passive perception could have picked up and trigger the trap. Players usually don't enjoy that, and it frequently leads to arguments like "well of course I meant to look at the drawers first, my PC is always looking for signs of danger!".

If they specifically describe examining the object/area where the trap is (whether or not they're doing it to check for traps), I would probably ask for an active check even though I have already checked their passive perception; as minnmass suggested this would be more likely to be investigation, but if it was perception that gives PCs whose passive check "failed" a chance to roll higher and notice.