the way i personally like to run insight checks is not to give the players straightforward answers like "they're lying" or "you think they're being honest", i like to factor in body language, "you noticed they did hesitate before answering", "they're avoiding eye contact". sort of like you're a body language / tone analyst. the better you roll, the more you notice that helps you come to a more accurate conclusion.
if the roll is average, i like to point things out but leave a certain space of uncertainty, "their brows knit together briefly, as if something about what you said troubled them, but you're not sure what."
average rolls mean you get a bit of insight, but it's not quite There yet. an average roll — "occasionally, their eyes stray and they glance over at something in the distance." — lets you pick up on certain things, but not quite what it means. is it a suspicious glance? a bitter one? a better roll might produce something like, "their eyes constantly stray towards that person in the distance. there's a bit of wistfulness to their expression." a natural 20, and bam, that's the look of love. "a gaze that cannot help but tend towards them like sheep flocking towards their shepard, or a harvest blooming under careful palms. it's the gaze of someone deeply in love."
lower rolls of course indicate that you can't read said person that well at all, "for a moment, you thought you saw a flash of an expression, but you didn't really catch what it was", "this person is a closed book. you can't glean anything about their reaction."
insight rolls are for insight, they serve to provide more guidance on how to read a certain character's body language / intentions. sometimes, there's nuances to it ! i can say, "they believe what they're saying", but does that necessarily mean that it's true? no !!! or a, "you do detect an element of honesty in their statement." what does that mean!!! does it mean that the npc is conflicted and doesn't quite know what the truth is, or does it mean that they're half lying, half telling the truth to you on purpose? who knows !!!
i will also always roll opposed for npcs in cases of deception. if an npc is actively trying to lie or hide something from you, it's your insight vs their deception (if they aren't lying or trying to hide anything from you, i will still roll anyway, just so the players can't go HEY YOU ROLLED THAT MEANS THEY'RE LYING!!!! i roll for show, i just don't use the result that i got !) this is partially because i'm not good at gaging DCs and i will often overshoot, but i also just enjoy that factor of variability in social interactions. it allows me to base a certain character's ability to lie in their stats rather than eyeballing it every time, and it allows for the concept of "sometimes people succeed at lying, and sometimes, they're just horribly bad at it at a given time". and what you glean from them depends on the difference between their roll and yours.
if the npc succeeds at lying / the PC fails at distinguishing a lie, i take low rolls not as a "well, you believe them, they're being honest" because that gives away the fact that they're lying, but more of a, "they paused a bit before speaking, but their answer sounds honest." why did they pause? was it because they were hesitating on telling the truth, but ultimately told it, or was it because they're lying to you?
the only time i really like to just flat out tell the pcs that they believe something incredibly stupid is if one of the pcs rolled like a natural one and the other rolled a natural 20. then i can start with the high roll, "they're definitely lying. you've seen this exact phony, bright, not meeting the eyes smile of con-men and salespeople trying to convince you to buy something at a ridiculously inflated price. it's a scam," before turning to the nat 1 and going, "you have never. ever. believed something more. 10000 gold pieces is definitely such an affordable price for this rock that was definitely not picked up from the ground just now. and by the way, this person reminds you a bit of your kind grandfather."
playing around with responses to rolls is so very fun and whimsy !!
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