So recently i got to join a campaign and i enjoy power gaming character creation so i made a character that has a passive perception and investigation of 20. Observant feat gives me a +5 to my perception and investigation which bumps me to 20. But theres a bit of contention between me and my GM. With a passive perception of 20, is rolling for perception ever truly needed? So far ive treated it as, “My character does a cursory glace around the room, whats going on with it.” It also prevent stealthy creatures from approaching us, but where do you draw the line between my character just being so observant and needing to make an active perception check.
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For a lot of things you may not need to roll but there will be situations where you need to focus on being perceptive that can require checks. Id think it’s mostly up to the DM to make it fit. I think from what I’ve read there’s mainly a difference between things you can notice passively and items you need to actively watch for. If I weren’t at work I’d read through perception to be sure and check it but can’t right now x.x, although afaik, Adam almost never uses the passive perception (maybe once/which is fine, unless he just ignores it because the cast has really low passives im not sure).
The way I’ve always understood it lies in what action your character is taking. For example if its your watch at night, you’re actively searching for danger: ie perception check. You look around a room searching for something: perception check. If you’re strolling through the woods and run into an ambush you weren’t looking for: passive perception.
In my mind passive and ‘active’ perception are two different things, passive perception is how good you are at noticing things out of the the corner of your eye. I hope that helps.
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Passive checks PHB p175 
:
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.
Here’s how to determine a character’s total for a
passive check:
10 + all modifiers that normally apply to the check
If the character has advantage on the check, add 5. For
disadvantage, subtract 5. The game refers to a passive
check total as a score.
Also see Errata p177
If you want to tryhard the passive perception/investigation, you have to use a vocabulary that doesn’t imply that you act. But as long as it is not an action that can be done repeatedly/passively, you are kind of fucked.
You can narratively force brut the passive side by describing your character as a paranoïd.
There is no “instinct” for that, but it can be a part of your “Personality trait” so that it is clear that your character is always 100% focused on his surrounding. It will help for the rule lawyering.
- In combat/charged situation, monsters will have to beat your Passive Perception (PP) with their Stealth/Sleight of hand check. It covers a large part of perception. It works with trap as well. When exploring a dungeon, players can say :
“This place looks suspicious, I check for trap” and do a perception roll. With maxed passive perception you don’t need it, because if your PP is higher than the trap DC you see it.
This is how far it goes for Perception. Note that Invisible creatures can still make noises, they may have advantage but PP still apply.
What you can’t do is arguing PP when sleeping, because you are unconcious.
- Investigation is trickier, because it only triggers if the GM has prepared hided clues. And if one of his players has “Observant”, he might metagame it. Passive Investigation (PI) triggers each time you see an object with hidden information on it. Such as a corpse, a room, fragments of a broken object etc. See Investigation PHB p178.
Clues work like traps, there is a DC to beat. (PI might help when reading a text with hidden clues that is not an elaborate cipher such as Thief Cant)
But if you don’t have any way to see that something is off, it doesn’t trigger unless your character is described as completly paranoïd.
For exemple, a Paladin would not trigger his/her PI in his own temple unless he succeded an Insight check on a fellow worshipper that would hide something from him/her. If there is nothing to be worried about, PI don’t trigger is what I want to imply.
I hope it helps
Just kill enough stuff to become a level 11 rogue aka a Sherlock Holmes level high functioning Murderhobo who never rolls under 10 on a d20 in an ability check whose skill he is (at least) proficient in. You know, just to make the GM go crazy. Passive what’s Passive to a rogue with Reliable Talent? The proper GM response is to drown you with many many useless but quite astute observations.
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On your point about being Paranoid, I play a knowledge hungry mage who is on the lookout for the secret to eternal life, so he’s always on the look out for any sort of clue. And being old and wise i’d imagine he’d be on the look out for many things.