Come to the Dark Side - We Have Cookies

 I want to talk about Star Wars for a moment. Specifically, the prequel trilogy. Anakin Skywalker starts out as a cute little kid with blond hair. Next movie, his hair is more of a caramel color. Next movie, it's dark brown. (Next movie, he's Darth Vader.) As his personality shifts from cute little flying ace to evil Sith Lord, his hair grows darker, as does his clothing. This happens to Luke, too.

In movies and books, there is almost always a physical manifestation of someone's inner darkness. Something about them, besides their spirit, grows darker. In real life, one's hair color won't tell you "Run away, I'm evil!" (Their sense of style might.) In literature and drama, there are no such limitations, especially in fantasy (or even sci-fi), where for all you know it IS possible for someone's physical appearance to change as he turns to the Dark Side. (This works in reverse, too. People change, usually becoming lighter, as they become good.) This is called physiognomy: what your appearance says about your personality.

Look at books you love. Look at your own writing, especially any darker pieces. How does your character's appearance change over time? The physical changes don't have to scream "EEEEEVIL!" - the lightsaber doesn't have to turn bright red. It could be subtler, like the changing hair color and robe color. The way the character talks (indeed, how much he talks) is just as telling. Bad guys use less contractions than good guys. It's arbitrary but it's a fact.

Pay attention to how your character shifts from good guy to bad guy or vice versa. (If you're not sadistic and morbid like me, and you don't have a ton of characters who start out good but succumb to their fatal flaws, then write the backstory for one of your bad guys and describe his fall to evil. No one starts out shooting puppies.) What changes about her appearance and speech that maybe even she doesn't notice, or you didn't notice until now?