>>63311934>Players have been betrated once so now they're highly suspicious of would-be traitors
Your players have learned to be smart, don't punish them for it with all these (admittedly fun) shenanigans >>63311953 >>63312043
. They'll only resent you more, crank up the paranoia, and resort to magical insight like Zone of Truth, thought-reading, etc.
However... you're missing something here. Think about this: the party triggers a trap in a dungeon. Now they waste hours searching from traps and you can't spring traps on them anymore! What's going on? How do you fix this?
Here's the thing: you need to issue a penalty for paranoia. Checking for traps means you're wasting time in a dangerous location, resulting in random encounters that drain the party's resources and leads them closer to failure.
But that's for dungeons. In a conversation, instead, trying to deduce whether a NPC is lying means you're focusing on their body language and appearing paranoid: you're basically shouting I DON'T TRUST YOU! And that should carry a risk of ruining your day, too. For example, if you DECIDE there's a chance the king is lying, and keep trying to analyze what he's saying, your reputation at the king's court drops sharply.
But traps and traitors are exactly the same in one aspect: in a good story, they're always foreshadowed. The flamethrower trap in the pharaoh's tomb leaves charred bones in a corner of the room. The deceitful king disappears at midnight according to rumors. (In general, try to include at least three clues. PCs are blind.)
Since your PCs are still traumatized from their first betrayal, try leaving clues about NPCs telling the TRUTH, too. Eventually they will learn to only roll Insight on *actually suspicious* NPCs, the way you'd only search for traps in a suspicious room. And you don't want your players to stop searching for traps ENTIRELY, do you? That's sensible, after all. You just want to give them a reason to only do it when it's smart, not all the time.