There is a story I will read every time I see it, no matter how many times I see it: Our Hero is undercover, and surrounded by badguys, but someone there, either someone else undercover or someone in trouble, begins to suspect Our Hero is not actually the villain they pretend to be. This seems to work best with fanfic, since the reader will already know Our Hero is not a villain, as in Gwendolyn Grace's
A Loaded God Complex (SPN, gen), or
Defect (SPN/CM, gen). It can also occur in pro-fic, but it's a bit harder to set up the dramatic irony where the reader knows the bad guy is sekritly good, but those around him or her don't. One that comes to mind is Timothy Zahn's Icarus Flight.
I'm fairly picky about this story, too: All cannot be Discovered. It's no good if, by the end of the story, the outsider has divined that Spike is in fact a vampire who went and got himself a soul because he was in love with the slayer, and is now attempting to make up for his several lifetimes of villainy. In fact, it's best (to me) if they decide he's a CIA agent, or a cyborg. But they should realize he is not the bad guy he pretends to be, and decide to trust him, even if only a little.
Perhaps this is the reason I find the opposite story ("Oh, hi," said Blair Sandburg, upon seeing Methos, the immortal he had met in grad school years ago, standing on his doorstep. (NOT EVEN MADE UP. REAL LIFE FICTIONAL EXAMPLE. (from memory))) so incredibly painful just to encounter. (Seriously. I twitch thinking about it.)
I think this is some kind of narrative kink, or emanation from the id on my part, but do other people have gen-bullet-proof kinks of this sort?