And this bypasses every single human instinct for detecting lies, because most GMs are not training themselves to have micro-expressions and secret tells that they’re lying. They are not LARPing this NPC. It is exactly the same person using the exact same tone of voice she used to explain the rules of the game.
So we just naturally assume that of course anything the NPC is telling us. After all, it is the exact same communication channel we used for learning the rules of the game, and you’re not off questioning those, are you?
Yeah, that’s definitely the issue, but I do think it’s something GMs should actively address, at least if you’re running a mystery or politics game where lies are important to the plot. Obviously, you don’t need to train yourself to do fake microexpressions, but intentionally introduce inconsistencies into the lying NPCs’ claims (whether internal inconsistencies or inconsistencies with other evidence).
Or there are other ways to signpost potential deception. One game I was in there was a priest in an order that vows to not lie, but the GM (not me) made a point that he refused to discuss specific issues with the party, instead foisting us off to a lay temple bureaucrat. Something that could an innocent explanation, but the fact that he kept insisting we need to talk to the guy who’s allowed to lie was a clue that he was hiding something.
You can’t totally fix the problem this way (which is why you should fallback on other strategies like making sure there are multiple ways to solve the mystery), but I think it helps to actively plan for it.
(skills and spells don’t really solve any of this, since you don’t have any context on when to use them - at best it’s a random chance to reveal a lie, but most NPCs will probably get a bit upset if you keep casting Detect Lies on them.)
Spells, yeah, though that depends on the exact magical rules of the setting. But Insight/Detect Lies skills are something that can happen subtly. Passively, even: just like the GM might roll a secret Listen check to let the party detect an ambush in the combat game, they can roll a secret Insight check when an NPC is lying.