I turned on the television early the other morning and caught The Deliberate Stranger, a movie about serial killer Ted Bundy, starring Mark Harmon as Bundy. Though this was made twenty years ago and I'd seen it before, I still found it darkly compelling and frightening. Harmon is one heck of an actor who portrayed the charming, intelligent killer without chewing up the scenery in the process. Creepy and disturbing are two adjectives to apply to his performance and the movie. Seeing this movie again made me want to call up my kids and review all the safety procedures drummed into their heads when they were younger.
Watching The Deliberate Stranger and seeing how this supposed all-American guy is revealed layer by layer as the sick killer he was will certainly give you some insight into characterization.
If you're writing suspense or have serial killers on the brain for a crime novel, take a look at this movie and read the book by Richard W. Larsen from which the movie was adapted.
Just don't do either at night, alone.