I don't know if I would or not. I wouldn't want any lines, that's for sure. If I were in FADE TO BLACK, I'd want to be one of the Weres in Donavan's house during his meeting. Just in the background. In PROMISES TO KEEP I'd ask to be one of the witches at Iris Falls Church. In MALEVOLENT DEAD I'd again want to be in the background, a Were or one of Damian's vamps. It would be an inside joke of sorts, one that my fans would pick up on.
I started thinking about this when I saw the premire of Eclipse. We went to the theater that was showing all three Twilight movies and for those of you who didn't know, Stephanie Meyer is in the Twilight movie. See Charlie over her left shoulder? It's really quick, and if you're not looking for it it's easily missed.
A lot of authors were in the movies that their books are based off of. I think Stephen King was in one (can't remember which right now). S.E. Hinton was in The Outsiders movie (she was the nurse.) Even Stan Lee is in most of the Marvel movies. Here's a picture of him in Daredevil:
So my question to my writer friends is this: if a movie based on your books was being made, would you make a cameo?