I can't do that. I have to watch the movie first, otherwise I get angry. For instance: Queen of the Vampires. I hated hated hated it because it did not follow the book at all. This way, if I watch the movie first, I have nothing to base it on.
I've read most of Stephen King's stuff, so most of the movies I hate, although I watched Green Mile before reading it, and I LOVE that movie.
I'm currently reading A Clockwork Orange, and the Road to McCarthy's.