This blog post was inspired by a tweet from Jeremy Crawford:
He talks about passive Perception more in a discussion on stealth in the Dragon Talk podcast; at the 22-minute mark, Jeremy says that the assumption is that a creature's passive Perception is always active, unless they are unconscious or otherwise unaware of their surroundings. Even when you fail an active Perception check, your passive Perception is always on; in essence, your active Perception check is an attempt to see if you can do better than your passive score.
The subject of passive checks comes up a lot in 5th Edition forums, so I love that Jeremy provided us with insight into the intent behind the intended uses of passive Perception.
The History of Passive Checks In D&D
An early form of passive checks was introduced in 3rd Edition:
Checks Without Rolls
A skill check represents an attempt to accomplish some goal, usually while under some sort of time pressure or distraction. Sometimes, though, a character can use a skill under more favorable conditions and eliminate the luck factor.
Taking 10: When your character is not being threatened or distracted, you may choose to take 10. Instead of rolling 1d20 for the skill check, calculate your result as if you had rolled a 10. For many routine tasks, taking 10 makes them automatically successful. Distractions or threats (such as combat) make it impossible for a character to take 10. In most cases, taking 10 is purely a safety measure—you know (or expect) that an average roll will succeed but fear that a poor roll might fail, so you elect to settle for the average roll (a 10). Taking 10 is especially useful in situations where a particularly high roll wouldn't help (such as using Climb to ascend a knotted rope, or using Heal to give a wounded PC long-term care).
Taking 20: When you have plenty of time (generally 2 minutes for a skill that can normally be checked in 1 round, one full-round action, or one standard action), you are faced with no threats or distractions, and the skill being attempted carries no penalties for failure, you can take 20. In other words, eventually you will get a 20 on 1d20 if you roll enough times. Instead of rolling 1d20 for the skill check, just calculate your result as if you had rolled a 20.
Taking 20 means you are trying until you get it right, and it assumes that you fail many times before succeeding. Taking 20 takes twenty times as long as making a single check would take.
Since taking 20 assumes that the character will fail many times before succeeding, if you did attempt to take 20 on a skill that carries penalties for failure, your character would automatically incur those penalties before he or she could complete the task. Common "take 20" skills include Escape Artist, Open Lock, and Search.
(Source: Player's Handbook (3rd Edition) p.65 or D20SRD.org: Using Skills)
Taking 10 and taking 20 were attempts to keep the game flowing by preventing unexpectedly bad rolls from derailing a scene or raising questions that undermine the plausibility of the game's story. (How do you explain a fighter with a Strength of 20 being unable to force open a rickety wooden door?)
4th Edition dropped the idea that a player could take 20 but retained the option of voluntarily taking 10 and added what 5th Edition would recognize as a passive check.
Checks without Rolls
In some situations, luck does not affect whether a skill check succeeds or fails. Two special types of checks reflect this fact: taking 10 and passive checks.
Taking 10 When creatures are not in a rush or not involved in an encounter or a skill challenge, they can choose to take 10 on a skill check. When a creature takes 10, its player doesn't roll a d20 for the skill check. Instead, the check result is determined as if the player had rolled a 10, meaning the result equals 10 + the creature's skill check modifier. For mundane tasks, taking 10 usually results in a success.
Passive Checks When creatures aren't actively using a skill, they're assumed to be taking 10 for any opposed checks using that skill. Doing so is called making a passive check. Passive checks are a convenient way to use creatures' skills without bogging the game down with die rolls.
For example, a group of adventurers is walking through an area without making Perception checks to look for danger, so each character is assumed to be using his or her passive Perception to notice hidden objects and creatures. If an adventurer's passive Perception beats a creature's Stealth check, the adventurer notices the creature without having to make a Perception check. If the adventurer's passive Perception and the creature's Stealth check are the same, the adventurer notices the creature if his or her Perception modifier is higher than the creature's Stealth modifier.
Passive checks are most commonly used for Perception checks and Insight checks, but the DM might also use a passive check for a skill such as Arcana or Dungeoneering to determine how much the characters know about a monster at the start of an encounter.
(Source: Rules Compendium (4th Edition) p.127)
The Passive Check in 5th Edition
Here's what the 5th Edition rules have to say about passive checks:
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.
For example, if a 1st-level character has a Wisdom of 15 and proficiency in Perception, he or she has a passive Wisdom (Perception) score of 14.
(Source: Player's Handbook (5th Edition) p.175 or Using Each Ability, D&D Player's Basic Rules)
5th Edition reframes the passive check as a tool for the DM, not the players. The 5th Edition rules call out several uses for passive Perception:
- Determining whether a creature hides successfully (Player's Handbook (5th Edition) p.177 or Using Each Ability, D&D Player's Basic Rules)
- Noticing threats while traveling (Player's Handbook (5th Edition) p.182 or Movement, D&D Player's Basic Rules)
- Finding secret and concealed doors (Dungeon Master's Guide (5th Edition) p.103-104)
- Detecting the presence of traps (Dungeon Master's Guide (5th Edition) p.121)
Using Passive Perception Won't Break Your Game
DMs who are used to running or playing in older editions may be reluctant to use passive Perception checks. They may feel that using passive Perception gives too much away to the players in a game where exploration is a key component.
But in D&D, the player characters are intended to be exceptional individuals. According to the Player's Handbook, an ability score of 10 or 11 is the normal human average. A character with a Wisdom or 10 or 11 translates to a passive Perception score of 10; if they are trained in Perception then their passive score goes to 12—that's not enough to beat a medium DC of 15, which is the baseline for most secret doors or traps.
A player who wants a generally-observant PC can set their Wisdom to 15, the highest you can get using the point buy method commonly used in Adventurers League games, which gets your passive score to 12 (or 14 if you are trained in Perception). Racial modifiers and feats can boost your passive Perception even higher: a +2 to Wisdom means your passive score can finally meet those DC 15 secret doors.
The Observant feat gives a PC a +5 bonus to your passive Perception but carries a big drawback: they are half as likely to beat their passive score on an active Wisdom (Perception) check because the player has to roll better than a 15.
...But It's Not Appropriate Everywhere
A high passive Perception does not make a creature omniscient. Without the aid of a spell or ability, you can't see creatures or objects that are invisible. Many illusion spells state that a creature must make an active Intelligence (Investigation) check to discern the truth of the illusion. Creatures such as gargoyles, treants, and animated objects have the False Appearance trait, which states that until or unless the creature moves, it is indistinguishable from a statue, tree, or regular object. Passive Perception doesn't help with those!
A number of traps or hazards might be discovered by a Wisdom (Perception) check: a covered pit, rope snare, or unstable section of wall might be easily recognized by a perceptive character. But many traps call for an Intelligence (Investigation) check to discover. Even though elements of the trap are visible—a row of holes in the hall for a dart trap, a large rock over a doorway that's rigged to collapse, or a series of nearly-imperceptible arcane glyphs—a character might not recognize the significance of the clues until it's too late.
Making Passive Perception Work For You
When a scene calls for a DC 13 Wisdom (Perception) check, it doesn't mean that a PC with a passive Perception of 15 is automatically aware of the detail as soon as they open the door. If you aren't making notes of the passive Perception scores of your PCs, you should start doing so. I would start with the general description of the scene, then zoom in to specific details as the characters would become aware of them in context.
When the characters enter the room:
You enter a small office. Large stacks of books cover every available surface, and papers stick out from every angle.
When the character moves to the desk:
As you look over the contents of the desk, a letter wedged between two pages of a heavy tome catches your eye.
I like this approach because it feels natural: the characters see what they see and hear what they hear. If no one could beat the DC 13 to find the letter, they may still choose to make an active search but until they do, that letter is just another unremarkable piece of paper.
You can use this method with secret doors. When the characters enter a room with a secret door:
The library old and dusty. A fine layer of dust covers everything, making it look gray and hazy.
When they come near the secret door:
As you move through the room, you see a faint ridge in the dust on one wall, going around the edge of what might be a concealed door.
You can convey sensory information such as sounds and smells in the same way. The party has arrived at a cabin in the woods looking for someone who is being pursued by bandits. When they reach the cabin:
The door hangs open from one hinge. It is clear from looking into the cabin that a violent struggle happened recently: large gashes appear on the walls, the furniture is smashed, and a bed is overturned against the wall.
When the party moves closer to the bed:
As you approach the bed, you hear a muffled cry appearing to come from under the floor. Someone is trying hard to stay hidden.
When it comes to detecting hidden creatures, the formula doesn't change much. A goblin moves behind some crates to set up a sneak attack but their Dexterity (Stealth) check is a 6, so:
The goblin breaks away from the attack and scampers behind a stack of crates. You lose him for a second, but you see the top of his cap poking out above the edge of a box.
What about a player who is attempting to hide from enemies? The temptation is to not tell the player whether their attempt was successful. How can the PC know what an enemy does or doesn't see? But I recommend letting the player know whether they failed. Withholding that information in order to mislead the player—to trick them into making a mistake—sets a bad dynamic at the table.
You dive behind a large rock outcropping, trying to hide from the orcs in order to set up an ambush. Unfortunately, one of them spots you as you peek around the edge and shouts in Orcish.
Running the game in this way builds a foundation of trust with the players that you will honestly and accurately describe what they know and experience. You establish a reputation as a reliable narrator, which helps your players make better choices. I've seen players who are compelled to thoroughly search every room, hall, chest, and barrel because another DM withheld information on the grounds that the players were at fault for not doing so. Anything that makes players suspicious of the DM's intentions is not healthy for the game, both at your table and as a whole.
If you aren't using passive Perception as a storytelling aid, give it a try at your next session!