[RPG] When is Passive Investigation used

dnd-5eskills

In the Sunless Citadel module, from Tales from the Yawning Portal, Passive Perception is mentioned about half a dozen times, but Passive Investigation isn't mentioned once.

However, both passives are increased by +5 by the Observant feat, and both are included when it comes to the Inquisitive rogue subclass in XGtE. In most mechanical and class mentions of Passive Perception vs. Passive Investigation, it seems they are supposed to be equals.

Passive Perception is used as the defensive stat for enemies attempting to use Stealth, but no such claim is made for Passive Investigation.

So when is Passive Investigation used, beyond being an automatic permanent minimum roll for an unmade active investigation check?

Personally, I like to use them as reactionary/defensive stats that the DM rolls against, for things that a character can miss (somebody passing by, a drop of water falling from the ceiling, knowing the intended target of a shot fired from an assassin). This is not, however, an official use of the passive skills. I'm interested in regular uses of the passive skills for the sake of this question.

Best Answer

The distinction between perception and investigation isn't that great

As I've seen it played out at table, passive Intelligence (Investigation) checks are most often called out by the DM when hidden things (secret doors, items hiding in plain sight, illusions, etc) are in the area that the PC can see or hear.

  • That said, most DM's I have played with default to Perception for a great deal of 'you notice this' more than any Intelligence-based skill. (A notable exception being our ToA DM, who uses a variety of ability based checks depending on the situation).

    Investigation. When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. (Basic Rules, p. 61).

The most common applications I have seen in game is a secret doors detection (a player had taken the Observant feat), the 'feet sticking out from under the curtains' deal (yeah, our PC spotted the thief who had run away) and a whole gamut of things that "just don't look right." Two instances stand out

  1. A 'you notice that the shadow is wrong based on where the light is coming from' as we entered a room and were examining the contents

  2. The old 'you notice a lack of a shadow' during an encounter with what was (we discovered soon after) an illusory creature.

    What I have also seen that fits very well is the use of passive Intelligence based checks that let the DM just advise one of the players "you recall (X)" without interrupting play for a roll.

    • This approach is going to be very table dependent. At some tables rolling a lot is just what is done, while at others it isn't.
    • The one I saw used most often was directed at that same player -- a passive Intelligence(Investigation) check for estimating the value of a precious item (gems and jewelry). I think the DM always told our rogue that for (1) ease/speed of play and (2) a rogue/thief would most likely be the one to pay close attention to that detail.

For a good illustration of the distinction between the two different ability checks, see @DaleM's answer.

About passive checks in general

As my friend Tanarii has suggested here, passive checks ...

... aren't for when a character isn't actively trying to do something.

They are for when a player isn't actively rolling a die. Either because they can't know a check is happening, or because it would require rolling over and over again.

Its passive on the part of the player, not the character. The character can be, even usually is, doing something active.

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