One of my favourite things that happens with some NPCs is when there is just someone who is straightforwardly good or bad (relative to the setting) and it makes the players doubtful.

I first had this happen during a Curse of Strahd game, which is obviously quite a bleak setting. I found it important to include moments and characters of levity to emphasise the darkness of the campaign. My players spent a long time being suspicious of an NPC who they reasoned surely must have some nefarious, ulterior motive, despite just being someone who found that helping people out was a small but powerful act of resistance in a bleak world. Eventually the players realised “oh no, what if this NPC is actually a good as he seems, and in fact, our suspicion is yet another way in which we are being corrupted by the darkness of this world?”.