When there is something for a character to notice, if their Passive Perception score would succeed at seeing it, the DM will tell them, in RP terms, that they have noticed that something is off, that they have a prickly feeling, or that something in their peripheral vision has attracted their attention. They can then roll a Perception Check, and/or inform the other characters that "something doesn't feel right," or keep it to themselves.
It follows that: If you have essentially unlimited time, you may use your Passive Perception score in lieu of rolling a Perception Check.
As a DM, I find Passive Perception pretty frustrating. Even a 1st level character, even built with Point Buy, can have a PP of 15, which in RAW, means they basically perceive anything they'd have to make a Perception Check of 15 to perceive. A Gelatinous Cube stays hidden in bright light with a DC of 15, which means why bother?
Here's the RAW on Perception Checks. (Basic Rules DDB link)
"Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door."
So, you use it to perceive, with whatever senses you have, something you might not see normally. Like a lurking Gelatinous Cube. Cool. If the player tells me that his Rogue stops at the dungeon intersection and peers carefully down the left hallway, making a Perception Check, I am happy to let them roll to see the Gelatinous Cube. Active Perception Checks for the win!
But, so, what about Passive Perception Checks?
Here's the RAW on Passive Checks. (Basic Rules DDB link)
"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."
So, it can be used when the DM doesn't want the characters to know there's something hidden to find. That's cool, I like that mechanic. Some people are much more perceptive than others. I also like how it covers repeated checks. If you have an hour to spend looking for a secret door, your PP is a good measure of whether you'll find it. It's a nice fallback from the "take ten" style rules, where if your character spends enough time they will eventually succeed at something. For this sort of task, there's not a guarantee, but if you're perceptive enough, eventually you'll find it.
But for "noticing a hidden monster," or for immediately, automatically finding a secret door as you walk past it, it annoys me and pokes my "that's not realistic" button. Rogue walks through the dungeon and is like, "Oh, look, there's a trap, and here's a secret door, and down that hall is a Gelatinous Cube."
So why not just tell the lead player to have his character make a Perception Check when there's something she might notice? Because then the whole party goes on alert, and even if I don't agree to the chorus of, "My guy makes a Perception Check, too!" it pretty much blows the secret.
So, instead, I am using the rule above.
Online, there is much discussion about this, and it is pretty convincing. Take the Cube example. "In bright light" is part of the description where the DC 15 is noted. So, in dim light, like in a dungeon, lit by flickering torches, or using shades-of-gray-only dark vision, the DC might be higher, say 20. That beats most PP scores, and would solve my problem, basically in RAW. What about if they are using a Light spell? The spell description says it casts "bright light in a 20-foot radius and dim light for an additional 20 feet." (Technically, torches have the same effect, RAW.) So, a DC 20 until they get within 20 feet, at which point it becomes a DC 15, and they suddenly see it. That's not so terrible, it probably gets surprise at that point, and can move (15') and engulf them (another 15'). Further, "in bright light" could also mean in daylight, and who isn't going to notice a ten foot tall wall of quivering gelatin in the bright light of day, however transparent it is?
So, sure. But there are a number of circumstances (like a secret door) where it doesn't work out so neatly, and adding DC to a check based on "circumstances" just feels manipulative to me, and open to abuse. So I use this rule, instead.
Here's the Cube example:
The Rogue is leading the party down the dungeon hall, with the second character in line holding a torch up high. They get to an intersection, and they decide to take the left passage. There's a Gelatinous Cube down the left passage. The Rogue's player says to me, "I'm checking for traps," and with a little dialogue we clarify that this means "I'm checking for traps in the floor and the walls and the ceiling as we walk." Not, you'll notice, "ahead." So the Rogue is scouring the immediate vicinity as the party travels (slowly) down the hall.
Until they are within 20', it stays hidden with a DC 20 in the dim light, but once they cross that line, it falls to DC 15. The Rogue's PP score is 16. As they travel down the hallway, I tell the Rogue to roll Perception to find any traps, and since there aren't any, it doesn't matter what he rolls.
Once they get within 20', I inform the Rogue that as he looks up to take the next step, he sees the Cube twenty feet ahead. The cube gets surprise, and attacks, etc.
Once they get within 20', I tell the Rogue, "You don't find any traps as you walk slowly down the hall, but something is off. You can't quite put your finger on it." Their PP sixth sense is tingling about the Cube.
The player tells the other characters, "Guys, something's not right here. Do you see anything?" And then the party could all make Perception checks, and get attacked as above in RAW. Or they could back away, trusting their perceptive Rogue, making Perception checks as they go. Either way, it's no worse than the RAW rule for the party, maybe better.
Or a secret door:
The party is passing through a guardroom in a dungeon, there is an exit door, and also a secret door with a DC of 15 to be spotted. The party is being chased by a group of orcs, and none of them are actively looking for secret doors. The same Rogue with a PP of 16 is in the party.
I inform the Rogue, "As you run through the room, you notice an imperfection in the wall on your left, it's a secret door!"
I inform the Rogue, "As you run through the room, there's something about the left wall that piques your interest." Then the player tells the party to stop, and goes to investigate the wall, making a Perception Check, that they fail. If they decide to spend some time, "Guys, I'm sure there's something here, go block the door!" then after a few rounds, the Rogue will find the door, as their PP is higher than the DC, however they rolled. But if they decide to keep going to stay ahead of the orcs, they don't take the time and have to give up on whatever it is. That's a better result: the party gets to make a decision about staying (because, duh, the players know there's something there), or moving on.
Here, the home rule is much richer, in my opinion. I could just change the DC, assuming they are rushing through the room ahead of the orcs and that makes it a more difficult task, and make it a DC 20, but what's the point? I might as well DM fiat it and just decide they aren't going to see the door. That seems unfair to the players who went for a higher PP on purpose.
Or being followed in a crowd:
The party is walking through a crowded marketplace. They are being followed by a member of the local Thieves' Guild. The party has absolutely no clue they have attracted any attention, and are oblivious, the players arguing about whether they can find dragon meat in the market. The spy is hiding in the crowd with a DC of 15. The Rogue has a PP of 16.
I inform the Rogue, "In one of your stops at a fruit stand you notice a small man in a green cloak is following you."
I inform the Rogue, "As you walk through the market, you get a sense that something is wrong." The Rogue could make a Perception Roll, actively looking for the tail, versus the tail's Stealth. The Rogue could tell me she is trying to be subtle, so I have her roll Stealth along with Perception, and we have a good old fashioned skills contest. Or the character could nudge the other players, and they all give it a try, except the dumb fighter doesn't bother to try to hide it, and tips off the tail. Again, it feels like a richer, more rewarding experience.
Again, I could add DC to the check because they haven't told me they are "looking around," but again, that seems to defeat the point of having a PP at all, and feels a little like railroading.
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