I'm a screenwriter as well as a novelist.
OK, nothing has been produced YET (notice the caps. I am determined) but I have had four of my novels optioned for screen (Madonna was considering one of my novels as her directorial debut for three whole days before she zipped off on tour), I got as far as principal photography with one script before the director ran off with the producer, and I've won some prizes. Right now, while waiting for my novel to come out, I'm writing a script for it because two producers expressed interest, but the process is so different. Most of the backstory has to go. Characters have to be slimmed and trimmed. Even the pacing is different.
And I love it. I begin thinking more visually, moving things around. Best of all, I get to spend more time with the characters.
I'm wondering, though, how other novelists/screenwriters handle this split. Can you work on a novel as well as a script at the same time? (Of course, you are talking to someone who panicked because she found herself working on two novels at the same time, something that made another writer friend remark, "If you had two kids instead of one, you would know this can feel normal.")