Actually, vampires have very little to do with Preacher (other than his best mate being a vampire - a classical one allergic to sun and drinking blood). The main story is about a disillusioned minister who gets possessed by the offspring of an unnatural celestial coupling, which grants him the power of command. Basically anything he says, you do. And if you piss him off, he might command you to count every grain of sand on a beach. He and his pals decided to go and find God, encountering all manner of insanity along the way. It was a rip-roaring yarn.
I wouldn't call this casting "against type." All I really recall of Tulip was that she was a junkie, abrasive, and competent.