Insight checks and what they mean

 Sometimes when a character is talking to an NPC, I as the GM will ask for an Insight check. This means his passive Insight noticed something as I'm trying to determine what or how much. This does not mean the rest of the party should start making rolls or even being suspicious of anything cause they aren't involved. Also if the players ask for insight checks without a good reason, I check what their passives are and tell them "well, your passive of XX didn't notice anything so far. Did you have some reason to think this person is lying to you?" and the player will have to pretty quickly come up with a good reason to ask. Only the successful character knows results, not the whole party. They need to separate this knowledge or you need to make it secret. They can't really share the information without looking like jackasses. RP accordingly. In addition, I will pretend to roll for deception when they roll insight, and then say something vague like "Yeah, they're absolutely convinced of what they're saying." Or "They're hard to read."

Think about what a successful Insight check means - it's not a "lie detector" in the sense of the PCs knowing the truth on the spot. What they can sense is someone's attempt to bluff or deceive - assuming they're doing so deliberately. With rolls that high, in fact, you may owe them better and more specific information that "you think they're lying". High Insight rolls should give actual (brace yourself) Insight into the subject. It's not just "they're lying". The NPC's body language and face should be giving away why they're not being honest.

"They seem to be holding back, afraid to reveal details."

"While you're sure he's definitely lying, you're equally sure he's terrified of something worse than your threats."

"You sense that they're leaving quite a bit out, largely because they seem to really dislike you and don't want you to hang around."

"They're clearly making up most if not all of the details, but they do seem genuinely distressed over something."

Don't forget about red herrings as well.

The nervous merchant prince, who is worried the recently assassinated king might hurt his business.

The spymaster who thinks the party is just as guilty as they think he is.

The head priest who can't stand the sight of blood.

And the King's brother who is worried someone might find out about his affair with the king's wife.

In reality though he simply choked on a bone.

Successful Insight should reveal cues and feelings of things, not the truths themselves. There's a spell for discerning truth. If that's not cast, they don't get to know truth/lie.

Insight checks to discern an NPC's honesty are one of the prime candidates for the DM to make the roll for the players, out of their view. The players should not know whether "She seems earnest and forthright" means they rolled a nat 20 against an honest person or a nat 1 against a liar. (But since I like to roll in the open we aren't going to do that.)

Note: most of this (if not all of this) is from another blog or a reddit post but this was so long ago I forget where and who. Sorry about that.