It really depends. In the case of Atonement, I realized that I had never read the novel and I'm a big fan of Ian McEwan, so I ran out and read it in the week before I went to see the movie. If it's a text that has intrigued me before and I haven't gotten around to reading it, I'll read it before.

If I've never heard of it (say beyond the title), I'm likely to wait until after.