I
Immortal Sun
Guest
What about how I handle it:
When the player declares he's trying to detect if the NPC is lying,* I tell him to roll an Insight check, and that if he beats the DC he learns if the NPC is lying or not. If the player beats the DC, all I need to say is "he's lying," or"he's not lying" whichever is true.
That's not telling the player what his character thinks, is it?
*Whether this is by goal/approach method or asking to make an insight check, fill in that blank whatever way you prefer.
Buuuuut, it's not really helpful and provides no useful contextual information. The player could have flipped a coin, or just picked a choice from the hat to determine if they believe the NPC is lying or not, because without the correct information, that's all they have belief that the NPC is lying.
The problem with the DM saying "he's lying" or "he's not lying" is that it doesn't answer what the NPC is lying about.
Sure, if the NPC is saying something like "The sky is blue." and the DM says "he's not lying" the PC can intuit that the sky is likely blue.
BUT if the NPC is saying something like "The King left on Tuesday with his wife and son to visit their family's hunting grounds, where the Queen's mother and father reside." Then responding with "hes lying" doesn't provide enough context to help the players out. The NPC could be lying about everything, or any of the following could also be true: the date the King left is wrong. The King didn't take his wife and son, those aren't his family's hunting grounds, and it isn't the Queen's mother who live there; and about a dozen combinations or variations thereof. Maybe he took the wife but not the son, or the son and not the wife. Or he went someplace else entirely where the Queen's mother and father do live, or somewhere else where neither live, or maybe the King didn't leave at all!
Certainly you can avoid this by avoiding NPCs giving out compound information. But quite honestly, if you're good at lying, this is how you do it. You make big bold claims that are utterly false "batteries are powered by the souls of the unborn" or you subtly alter the truth in a way so it remains believable, but the end result is faulty information. The King actually left Tuesday, he took his Mistress, not the Queen as they left the day before to go to her parents. The King went to see the witch of the woods because he accidentally knocked up his mistress and needs an out.
You've told the player they know the NPC is lying....but you haven't told the player how they know the NPC is lying. And if the party just starts accusing people of note of lying, they're more likely to end up dead than furthering the story. I mean, unless you're soft-balling intrigue and how wealthy and powerful nobles would react to a bunch of dirty scoundrels accusing them of misdeeds.
Now, granted, maybe you're just not going that deep on the lies. Maybe it is all simple stuff and well, okay your approach works. The player can reverse engineer the lie to determine what the truth actually is and therefore retroactively have the knowledge to know the NPC is lying. Maybe you're not doing plot and poison. Okay cool beans.
But this is one of those moments where the application of Insight would have to be handled like Quantum Physics. Regular physics just doesn't apply on a sufficiently deep level. Insight may certainly be one of those skills.
Oh and to note, I'm not terribly worried about the "telling the player what their character thinks" note. In certain circumstances, I think it's fine to do it, and others not.