Part of an ongoing (if occasional) series of D&D 5e Rules notes.

I’ve found Insight rules terribly underutilized in the D&D games I’ve been in. Wisdom (Insight) [PHB 178] is essentially Perception for personalities.

Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone’s next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.

Examples of using Insight

Bob the Tailor is a town elder, who’s fluttering around trying to keep the party from the abandoned mine outside the town. It would be useful to know where his fear is oriented — toward the party, toward the town, or toward himself. Is he lying when he talks about the mysterious music people have heard in the area? If we we say we’re going to the mine anyway, does his fear spike — or is it anger? Is his smile when he sees the constable approaching the confidence of seeing an approaching ally, or a deceptive cover for terror at being discovered? Insight can help with all that.

In other cases, you might use Insight to figure out if the guy you’re gambling with is confident in his hand? How does he feel about that last card he drew?  Is your date having a good time? Sure, she says she likes that roast beast you ordered for her … but how is she really feeling?

If someone’s trying to actively resist others using their Insight against them, they usually roll Charisma (Deception). (This is a case where one could easily use other base states for the Deception role, however — an academic using Intelligence (Deception) in hiding their bias in a paper, for example, or someone using Strength (Deception) to hide how incredibly freaking heavy that chest of gold is.)

But rather than active rolls, this is also a case where passive skills come into play — the GM can consider passive Insight (or another’s passive Deception) to give give unsolicited clues about “He’s behaving a little twitchy,” or “He seems genuinely worried about you,” or even “You notice she seems attracted to the barkeep.”

Limitations of Insight

It does have limitations. It can indicate that someone is lying — but not necessarily what they are lying about,  or why they are lying, or what the truth is. People lie, after all, for a lot of reasons. Insight might tell you that the city guard you’re talking with still seems highly suspicious of you after your story … but it won’t tell you that he’s going to let his friends know to keep an eye out on you, or that he’s going to try to ambush you later on.

Insight gives you, well, insight into underlying feeling, reactions, etc., but not necessarily why they are reacting that way. Is the guard at the door speaking a bit flatly when he tells you about how great a guy the grand vizier is? Yeah, you can pick that up with Insight, but it’s going to be more difficult (i.e., take more time and questions and other actions) to tell if the change because of some sort of loyalty spell, or from fear that the vizier’s secret police are monitoring him, or even just boredom with people pumping him for information about the vizier.

The nature of Insight — picking up on tells, physical and verbal expressions, etc. — requires you have a way of perceiving and interpreting such things. Dealing with the human barkeep at the tavern is one thing. Trying to read the body language of a gelatinous cube is another.

Even in less extreme situations, Insight might be hampered by unfamiliarity with the target’s customs and culture: shouting and waving around your spear might be an expression of hostility by this never-before-met humanoid, or it might be a ritualized greeting, or a mating display. Insight might still work, but less reliably.

Remote use of Insight

Finally, Insight can be used without the target standing in front of you — picking out a great gift for your girlfriend (or for the prince) based on what they’ve enjoyed in the past, or figuring out how the savage Orcish war leader you keep encountering is likely to attack the city or respond to various counters. I’d probably use a normal Difficulty DC as the opposition, and familiarity (or lack thereof) with the target would be a key in determining how difficult the estimate was.

Overusing Insight

Because it deals with interpersonal relationships, Insight can be easily abused or overused. Overuse of  Insight is a bit like overuse of Perception (“I evaluate every person in the bar” is like “I search every room thoroughly”); it’s doable, but should carry some costs (time being a major one, but also something like the likelihood someone is going to catch you staring at them — or their love interest — and take offense).

Overuse also takes away a bit from Role Playing. The DM should be able to use passives to feed needed clues to the players about how people are behaving without their insisting on active Insight rolls, just as they feed visual prompts in the normal course of things rather than  players requiring active Perception  roles as they walk through town.

Who rolls Insight?

Note: Insight is one of those skills (like Perception, etc.) where sometimes it makes more sense for for the DM to roll it for a character, to determine if you can figure out something, can’t figure out something, or are deceived in your insight about something.