I wanted to write about NPC betrayal. I've got an unorthodox method of npc betrayal in my campaign at the moment and got to wondering how I would handle it other ways.
The npc in question is an early patron of the characters, a wizard specifically. The reason its unorthodox in the way i'm handling it is because the players know he has been replaced by a doppleganger, as they also run evil characters in the same campaign, something i've written about a little before. Whats great is that they know he's bad news, in meta game, but are fantastic enough role-players that they can push the story forward whilst using their good characters without using that information, and seem to have a much deeper emotional response to the character, and in fact, all the characters they come across in their dual lives at the table. Whats a real shame for the group however, is there will be no 'Aha!' moment. No reveal. Which kinda sucks.
So that got me to thinking how would I handle the turncoat thing differently, usually I'll set the story-line ball rolling and react to where it goes. I like to tell the story that players ask for. So I don't think having one enemy turncoat would be as story effective as having several potential turncoats. That way, when the players are at their most vulnerable - I can choose the npc who is under the least amount of suspicion and reveal him as a double agent for the maximum effect.
If intrigue is a focus of the campaign at any given time, i'll ensure that i'm giving out the vibes from several key npc's, then start letting clues drop - eventually they find out there's a mole - then they'll hopefully try and smoke them out. Perhaps discover a nefarious plot, and just in time, foil it or watch it go off.