Is it fair to only consider the result of "passive" Perception checks (same as Taking 10) for PCs to detect things, unless the player actively expresses attention to something (in which case a roll is made)?
I've started distinguishing "active" perception from "passive" perception recently after reading about the concept in Mutants & Masterminds 3e (I think it also exists in D&D 4e). It's mostly a way, as a GM, to avoid rolling constantly. You can even come up with a simple formula to pre-emptively define the distance at which some things are detected.
I also tend to think it's more believable as no one is constantly 100% aware (which getting a 20 on a roll may represent). Well, since this is fantasy, at least no one without a special power/feat is 🙂
Thus, when a player has suspicions about a character or expressively pays attention to something, that's when I make rolls and the character has a chance to go beyond the average 10. I also consider a minimum die roll of 10 (why would you do worse than routine when you're actually paying attention?).
Of note: I only do this during quiet times (when Taking 10 is appropriate). During combat, noticing something requires a full check and die results from 1 to 20 are taken into account. Also, I currently use this for Perception and Sense Motive, but may apply it to other things like Spellcraft (routinely identifying low level spells thrown at you).
So, to repeat the question and elaborate:
Is it fair to do things this way? Am I unbalancing some aspect of the game I'm unaware of by doing this? Could this lead to other problems down the way? For example, I have no idea if the system expects characters to be able, more often than not, to get 10-20 on the die for such checks. Maybe my passive PCs will miss way more than they should with this method?
I'm not interested in the specific topic of players saying they're inspecting everything all the time as I believe this to be a whole topic of its own and it has been treated in various other questions on this website and elsewhere already. Anything else than this issue?
Best Answer
It is fair and you are not throwing imbalance in the game.
Well... maybe a 2,5% imbalance on the long run (the expected value for multiple d20 rolls is 10.5).
In my experience, Perception is one of the most used skills. Also, it is - more often than not - a passive skill (the GM asks for a Perception check). However, the mere asking for a check puts the players on their toes, and metagaming arises even unintentionally on a failure.
Adopting the rule M&M and 4th edition use by default is, in my opinion, an improvement in gameplay: it saves time and avoids metagame.
By letting players make Perception checks when they ask for, and using passive Perception for things that they may not be aware of you can speed up the game and preparation time (for example, you can decide beforehand if the party is surprised or not, according to the Stealth check of their foes).
As a GM, I'll usually go further and use passive Knowledge (whatever) for giving them instant information on the stranger phenomena or monsters that they may happen to encounter and a passive Sense Motive against bluffing NPCs. In both cases I keep the opportunity to let them roll if the player explicitly ask for.