You know what’s spooky? Someone whispering in the dark: “Do you trust me?”
In fact, asking for trust in any way is generally off-putting.
Have a NPC, for no apparent reason, ask the PCs for trust. Make it someone who the PCs have no choice but to depend on (guide, patron, provider of bail). Whether or not the NPC is actually treacherous, the PCs will start casting a lot of glances over their shoulders.
It works the other way too. Right before a combat, have an ally NPC say, “Whatever I seem to be doing, trust me.” Then, during the battle, he switches sides and starts attacking the PCs. “Maybe, just maybe,” the PCs will think, “this is part of some plan to help us.” Is it, or is the NPC just an evil jerk? Who knows!
PC uncertainty in combat can be a lot of fun (for the DM). It works especially well with time limits on how long each PC can spend thinking about their turn. Time for frenzied panic!