Sometimes you might have this happen. Some NPC spills his guts dropping off some key information to the party. You get one guy that grabs a D20 and says, ‘I’m gonna see if he’s lying with Insight.’ Nope. Hold on. Back up there, buddy.
See I don’t buy insight acting like a default lie detector, where players wave their hand an automatically read untruthful thoughts. Insight is also about reading social situations. Great insight allows a person to recognize the two people chatting civilly over in the corner really cannot stand each other. Insight allows a player to hear the slight strain in an NPC’s voice saying things are fine, and recognize that subtle shift of their eyes to some burly thugs nearby.
Want to go all out with insight being a BS detector like Christopher Walken in True Romance? I’ll be rolling a bluff check against it. As a DM you should always be rolling bluff checks to counter the ‘insight = truth meter’ that players pull out. If they blow it, I say they believe them. If they beat out the bluff check, I rarely say, ‘You think he is telling the truth.’ I always try to obfuscate the result with, ‘You think he’s holding something back’, or ‘You see him lick his lips and give a smirk.’ I rarely ever give a black and white answer to passed insight checks if they are just seeking a truthful answer.
To me, insight is more about reading the subtle body language of people. I like to frequently give clues to the NPC’s mindset more than if they are simply telling the truth. Does the person have a pleading look in their eye when they beg the group for help? Do the player’s see the Duke slightly roll his eyes when he thanks them for dispatching the orc marauders? Does the group see the inn keeper tense up when they approach?
I use passive insight checks a lot to allow a group to read an NPC’s motivation. Sometimes it is a much more effective hook having a NPC say one thing, but his body posture or mannerisms give a completely different impression. I find it frequently sparks that investigative process where the players slowly poke around an NPC through dialog. It is much more interactive than just having a PC roll a D20 and see if the NPC is BSing.
I also allow insight to get clues on other skill checks. Sure some PC might ace the diplomacy check, but he just successfully made the flunky patrol guard allow them to pass. It’s the seasoned sergeant of the guard that really decides who gets into the king’s court. Insight is the key skill to walk into the room and assess who are the likely people with some authority, seem knowledgeable, or are well respected by others.
When do I have PCs use insight for a yes/no answer? Illusions. Yup, most folks forget insight is the key skill in disbelieving illusions. Since the players have a tool at their disposal to counter illusions, don’t be afraid to pull them out. In a magical fantasy setting, there should be plenty of opportunities to pull out the illusion card. It doesn’t necessarily have to be diabolical either. Maybe a merchant uses a simple illusion to make his wares look plentiful. How about an enchantment to give an elder noblewoman a slightly youthful appearance (think of it as gnome magic botox)?
So the next time a player quickly rolls a D20 for an insight check, don’t just respond with a yes/no to queries for the truth. Tell the player he thinks an NPC is holding something back. State the NPC appears to be sweating profusely as he stumbles over his words. Get away from treating insight as just a lie detector.