Hiya!
Inspired by the Cloak of Elvenkind thread, I've finally decided to address my biggest source of confusion in 5e. Almost seven years in, and I still get occasionally confused about when Passive Perception is used in lieu of normal Perception. It seems to me that it would be when walking by secret doors, noticing traps or ambushes—sort of like the elf's ability to notice secret doors in AD&D, but that begs the question of when does "active" Perception get used and why?) Then there's Investigation. When does it come into play instead of using Perception? When I search a desk, am I using Perception or Investigation? Lastly, why are the rules for these things so virtually non-existent?
So, okay comunity, what are your thoughts on the matter? How have you parsed these things? And where upon the rules do you base your interpretation upon?
DM: "You travel down the 5' wide stone corridor. The walls are slick with water and slimy-moss, the floor, even worse." [secret door here with DC 14; looks at PC's Passive Perception of 12]
Player: "OK, I'll keep walking, but at half rate to keep my footing"
DM: [thinking that moving at half rate is worth a +2 to Passive Perception] "At about 40' down the corridor, something catches your attention. A section of the wall on you right, about 9 or 10 feet of it, seems to have less of that slimy-moss on it"
Player: "Really? Hmmm...I'll stop and raise the lantern a bit to get a better or closer look...do I notice anything?"
DM: "What's your Perception?"
Player: "It's +2"
DM: "Right...[rolls d20, gets 17, +2 is 19]. Now that you are looking, you notice that there are distinct 'lines', pretty much straight, on the left and right of a 6' section. The floor here, by that section, also seems to be cleared of most slime...not nearly as thick. Probably some kind of Secret Door"
Player: "Ha! Ok, I'm going to start looking for bricks or rocks in the wall that are different colours, or smoother, or something. I'm looking to see if it can open from this side"
DM: "You press various rocks that catch your attention...what's your Investigation? Make a check for me..."
Player: "Ok...I got a total of... 15"
DM: "That'll do it! There are two rocks, one on the door, one on the wall next to it; pressing both at the same time results in a satisfying 'CLICK!' and the door opens towards you about one inch. The smell of rotting flesh spills out, choking your nostrils and throat....what are you doing now...?"
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That is how I run Passive and Active Perception, and Investigation.
Passive it to "notice something that gets your attention". Active when you try and discern what it was that grabbed your attention. Investigation is when you find out exactly what grabbed your attention and now you want to do something about it.
Oh, and yeah, I tend to roll the PC's Active Perception checks. Sometimes others as well, when the binary high/low of the system would give too much info away (re: "Make a Perception"... "Whoot!, with my silly adjustment I have 28!"... "Oh. Uh...you don't notice anything" ... "Cool guys! This area is totally safe...lets camp!"
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^_^
Paul L. Ming