Swarmkeeper
Hero
If a PC asks to roll Insight against an NPC telling the truth...
Slight point of order here... I've found the game works best as intended: the players don't call for rolls - the DM calls for a roll only if there is a chance of success or failure and there is a meaningful consequence to failure to the PCs' actions. So like this:
I ask the player what the character is doing to determine whether or not the NPC is telling the truth and then determine the DC based on the approach.
Of course, with no meaningful consequence of failure, I'd go with this:
I likely don't set a DC. The character succeeds, no roll, perhaps because the truthful NPC exhibits no body language, speech habit, or change in mannerisms that suggest a deception.
And keep in mind:
Indeed, the player has every right to roleplay their character as naive or paranoid. No specific die roll needed to do so.A player is always the final arbiter as to whether their character is persuaded or intimidated.
And more good advice on these themes here:
...If the players want to find out if they are lying, they have to catch the npc's on falsehoods or inconsistencies in their story, rather than a lucky dice roll. To me that is far more exciting roleplaying wise.
...
When a player searches a room, I ask them what they are looking for. If they want to disable a trap, I ask them how they want to disable it. I do the same with social checks. You can't just tell a person is lying, but perhaps you can tell if he's nervous? Or whether the things he says are factual? The player needs to state an approach, and then I determine if it's an auto-fail, auto-success or a dice roll.
....
So when an npc says he has the players surrounded, I don't want my players to say "Can I make an insight check to tell if he's lying?". Instead I want them to say something like "Does it look like he has a lot of allies in the bar?" or "Do I see other people with weapons?" or "Does the npc seem nervous?" or "How does the rest of the tavern respond to these threats?"
Not to say any of this is simple - after years of DMing 5e, I'm still working to master this style of play. Players at our tables (especially those with habits from older editions) still try to "press buttons" on their character sheet, invoking a skill mechanic while asking to roll OR even just rolling and announcing a result. That's not just on them, though. I need to strive to do better at setting the scenes and awarding auto-successes for clever play to keep the positive feedback loop going - then the players will more often tell me approaches and goals rather than grabbing dice first and asking questions later.