DM Advice: Always roll for Deception
Deception at the table is a tricky thing to handle. At least it has been for me. A deceptive NPC or monster may be subtle or overt. They may outright lie and attempt to trick the party into doing something that would harm them. Or, it could be as benign as withholding useful information and feigning ignorance. All the same, playing a deceptive character requires a touch of foresight, general understanding of the NPC's motivations, and a strong poker face.
As many of us Dungeon Masters are aware, players (and by extension, their characters) can be incredibly suspicious. They will find hidden motives and deceit anywhere. Even in the kindly tavern keeper that really, truly, is just a kindly tavern keeper. When the DM knows for a fact that the tavern keeper is a swell person it seems pointless to roll a Deception against the player's Insight. The Insight check practically resolves itself. "Yep, the tavern keeper seems honest." In fact, it may be tempting to forego an Insight check altogether when it comes to an earnest NPC, especially if we're following the advice of my previous article "Roll at your own risk."
Insight is different, though. Insight is special. There's one easy "trick" we can implement effortlessly to make running deception a wee bit easier. When a player asks to Insight check an NPC, always allow it and always roll a Deception behind the screen. No matter how sweet and honest that NPC is, pretend for a moment they're not. Roll that d20, glance at the number as if doing mental math—no matter how irrelevant it is—and proceed from there.
This will keep players guessing and make it less obvious when the Dungeon Master really is rolling for Deception. If we only roll a Deception contest when the NPC is actually being deceptive then it will be a dead give away the second the d20 hits the table. It's a cue to the players and it will heighten their suspicion. Instead, if we roll Deception for every Insight check we remove that cue entirely. This trains the party to ignore the clatter of the d20 and focus on the results of the check. While this may seem obvious, it took me a few bad attempts at Deception to come to this conclusion. Hopefully this simple tactic will help other DMs at their table.
There's more to it, though. Even if an NPC isn't being deceptive and the DM throws out whatever was rolled for the Deception contest—we still need to take into account the rolled Insight value. If it is low, it may not be obvious to the character that the NPC is honest and well meaning. If it is high enough, the PC may have a very strong read on the NPC and feel confident they can be trusted.
Insight Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone’s next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.
PHB. pg 178 | Using Ability Scores
It's up to the Dungeon Master to meaningfully translate the resulting Insight check into something the player can make deductions from. Especially in the case of a real Insight vs Deception, where the NPC or monster under scrutiny does have something to hide. As I mentioned before, a truly low Insight against an honest NPC indicates the character failed to read the open body language, failed to intuit their sincerity, or failed to make any useful deduction from what they observed. In these cases it is okay to go with the blanket statement "from what you can tell, they seem earnest" or, if you wish to sow doubt, "you don't deduce anything."
It's a balancing act that I'm still working on at my own table. There's a lot of variables to consider when trying to play a deceptive NPC. I'd like to delve more into that at a later time. Today's takeaway is simple. Always roll for Deception!