There are moments that happen in works of fiction where one character reads another character like a book. A conversation happens, and as the characters separate one explains a whole slew of things they took from the conversation that were never discussed. To the viewer it looks like they discussed a grocery trip. However, from discussing that grocery trip the hero - or sometimes villain - knows that the secret plan kicks off on Tuesday and starts at Steve's house. How do they do that?
While my above example is hyperbolic, it hopefully go the point across. Most systems have a way of pulling something like that off, and it comes in the form of whatever skill covers reading a person. In D&D 5e this is the 'Insight' skill. In Legend of the Five Rings (at least before their 5e) it was the Courtier skill. I believe the Dark Heresy system uses Scrutiny, while FFG's Star Wars uses several skills for everything depending on the character's approach and the situation.
These skills cover the ability of the characters to read people, but there are two common problems I find most games - and groups - run into.
1. How do I know when I should be using this skill?
2. What can I discern from interacting with someone?
How Do I Know When?
The normal answer - and the one most systems have - for this question is that you - the player - should tell the GM when you want to scrutinize someone to see what else you are gleaning from a conversation. However, this involves the player needing to recognize the need in a situation and unlike their character the player may not be well versed in this. My character is a super spy that makes James Bond look like an amateur. I am not. We just accept this for things like infiltration and hacking, so why am I reliant on my ability to discern someone is being "shady" or "hiding" - and even worse - why am I reliant on my GM's ability to portray someone being 'shady' or 'hiding something' and pick up on how that is different from the GM just trying to improv a character they maybe aren't comfortable with or weren't prepped to roleplay?
On the inverse, if the GM just volunteers "you should roll insight" you can tip the hand of when things are going off kilter when that knowledge should not necessarily be conveyed. It's like asking someone for a perception check and then saying "they don't notice anything." Sure, the players know they're supposed to go forward thinking everything is fine. But they also know that a perception check was just failed which means they missed something.
This is one of the places where I really like 5e's "Passive" score. By making a roll for the NPC against the passive insight of the PCs, I can then volunteer extra stuff they glean from interactions. I can do this when it is important, and when it is unimportant. It can happen with small things - like haggling over a box of healing potions - or big things like talking with the King about the advancing army on the castle and why he doesn't seem as shocked or dismayed as everyone else.
The passive score also lets me copy from naval warfare on 'passive' vs. 'active' scans. A character's passive insight is what they just pick up from normal interactions. Sherlock Holmes sitting in a restaurant cleaning his utensils. He's not trying to read the room, but he can't help but pick up all the details because of how perceptive and insightful he is. Then, when a player tells me they'd like to roll insight, they are 'actively' assessing. This is where - narratively - you get the long pause as the character's eyes narrow and they visibly weigh the information they have. It is a moment that, if nothing else, conveys a brief "is this person fucking with me/on the level?"
How do you use a passive score for non 5e systems? Well a passive score just assumes the roll of a 10, which is the average roll of a D20 when you round down. So take the average roll for the character and just use that for the PC's passive score. After all, on average that is what they'll get.
What Can I Discern?
What one can read on the other hand is a bit trickier. Fiction, and even real life, is full of people who will glance at someone and then tell you that they're lying. Sometimes it is explained as human psychology. Sometimes it is a 'tell' like in poker. Sometimes it is because the character knows something that conflicts with the story told.
Beyond that really depends on a few factors. One of which is how well does the PC know the character in question? The better you know someone, the more you read on them. Picking up that your friend Michelle has been sitting in the back of the room quietly for an hour means different things depending on how well you know Michelle and her tendency to sit quietly for an hour present but mostly disengaged from the room.
Context is king here, but as the GM also ask yourself something: Does the insight serve the game better by being the answer to the question, or by leading to another question?
While answering that question, also keep in mind other information the player may already have. Insight is as much about reading people as it is about putting together disparate information (as is scrutiny, and those other skills.)
Also consider the roll or degrees of success. Insight, and Perception, are two places I very much enjoy using a sliding scale where the higher the degree of success the more information given.
As an example of how this could break down consider the following responses to an insight check (passive or active) to watching an NPC react to some visual evidence of a person.
- You definitely catch a hint of recognition in the person's eyes when they see the person in the footage.
- You catch a hint of recognition and a shifting of a hand to cover the mouth suggesting familiar, even fond recognition along with surprise. Like a long lost loved one.
- You catch a hint of recognition, and note some striking similarities in the eyes and other aspects of the face between this person and the person in the picture.
- You catch a hint of recognition in the eyes, but it is gone quickly. The shift in stance speaks of military training, and not the 'basic grunt' stuff either.