Say you are running a game. Encounters with monsters can be fun, but you decide to add a little mystery or intrigue, something that will start early and then be revealed later. A traitor perhaps, that helps the PCs but is later revealed to be a backstabbing jerk, plotting domination or ruination of something the PCs hold dear. You introduce the traitor as a silver tongued noble, who speaks to the PCs about doing good for the town/city/nation, but one of the PCs has the ability (a spell perhaps, or some other power) to instinctively know when someone is lying, exposes the traitor, and the PCs avoid weeks or months working with the bad guy. Your plot is suddenly reduced to nothing.
A lot of games have this ability to instantly know when someone is being untruthful, even if it is a spell, or requires a roll or a cost of some kind. Even if there’s not an ability to know when someone is lying, the players will still have an ability to force their opponent to speak the truth, which pretty much comes down to the same thing, if used correctly. I haven’t played D&D for years but I still remember the spell Zone of Truth, which changed my plot dramatically. In Scion, the ability to know when someone is lying is without cost or roll, and is absolute. There are no exceptions named in the description; not even Loki, god of tricksters, can lie to the person with that ability without being caught.
In the 3rd edition of Exalted, which I’m currently running, one PC has this ability to know when something suspicious is going on. When the situation calls for it, the player is told to make the appropriate roll. This is not the same as telling automatically if someone is lying, but it can counter any character trying to hide their intentions or identity, which again counters the mystery.
I could go on and talk about why these abilities are even included in the game, essentially breaking the mysterious sides of stories, while there are few or no abilities as accessible to instantly end a fight with a single strike. Personally, I think it’s because social and mental mechanics aren’t given as much detail as physical mechanics. But I could be wrong. This post is not about why the abilities exist, but how to work around them.
First of all, you need to understand the ability’s limits. Zone of Truth can only force you to tell the truth, not to actually tell them what they want to hear. If you’ve seen Shrek the 3rd, think about the scene where Prince Charming is interrogating Pinocchio. Pinocchio is not lying, he’s just talking around the question, telling the truth about not knowing where Shrek has not gone or where he is not supposedly supposed to not be. Also, if the target knows they are being interrogated, they could refuse to answer (“I don’t want to talk about it”).
If an ability can tell when someone is telling a lie or only half the truth, make sure if it can tell the difference. If the PCs have this ability and listen to the captain of the guards telling the thrilling story of how he fought three giant orcs while someone opened the gate to let the orcish horde into the city, their lying-detector might start ringing, but they won’t know if the captain let the horde in, if he really fought three or fewer orcs, or if they were as big as he described them, or if the captain was in truth drunk and passed out while the orcs entered through the gate he forgot to lock. In short, even if the captain is lying, he may still not be the traitor which the PCs are looking for.
Finally, abilities that can detect lies will likely not work if the subject it is used on really believes that they are telling the truth. The evil noble really believes they are trying to save the city, even if they need to destroy it and rebuild it anew to do so, which would lead to the death of hundreds. If the PCs ask them if they are really trying to save the city, their mystical lie-detector won’t reveal anything if they answer “yes”. Lies through proxies is also effective to avoid automatic lie-detection. If the noble has convinced a henchman of their lies, interrogating the henchman won’t reveal any lies, because as far as the henchman knows, he’s not lying.
If the PCs could still detect the lies, it would mean that truth is a mystical force that can be tapped into, and the PCs could thus discern facts from fiction, even unintended fiction. It would not be impossible in the fantasy realms we all enjoy, but it would affect what sort of stories could be told in that particular world. If you want to explore that option, I think it would be cool, but until then, work around the mystery-breaking abilities.