Christa Miller brought up a good point earlier, that writers are sometimes like method actors. We try to get inside our characters' heads, write them from the inside out--in essence, becoming that character.
I'm interested to know how other writers approach Evil. When you write a villain, whether it be a serial killer, a jealous stalker, a terrorist, a disgruntled employee bent on revenge, etc., how do you go about getting inside that character's head? Is it merely a matter of research, or do the best writers have a true dark side they can tap into?