One of My Stories

So I was just working on a scene for a story that I'm writing. It's coming along okay, but one of the things I could really do without is my so-called "writing process." I'll get an idea for a story, jot down some notes, think up a few names, and then start typing. Then after, like, one chapter, I get an idea for the sequel! I have one particular situation where I know what's gonna happen in the sequel to a story I haven't even written yet! Sound annoying? It is, but at least it keeps the juices flowing, you know?
Anyway, about this story, my protagonist has a pretty dark, tragic past (don't they all?), but I'm having a little trouble developing the enemy's exact character. I know I want them to be the enemy, I know they are despicable, cunning, and would make fellow-Thespians proud. But for this particular instance, I'm not sure how to show that evilness, the traitor beneath that delicate façade. Grrr. I want this to work --it has to work. They are in a market and the enemy has just purchased something apparently innocent, but when confronted about it they give a very weak defense. That's the gist of the scene, now I need the details. The enemy must have some kind of tell, however small it may be. And the protagonist must see this cruel fabrication. She must!

Ideas anyone?

-Grace