from those having experience in writing screenplays. Never having done it, how difficult is it to convert a book to a screenplay?
I've written spec scripts but never tried to convert one. My guess is, the answer varies from novel to novel, with length probably being the most common challenge: A novel can run 3 to 4 hundred manuscript pages or more; a screenplay is limited to maybe 120 formatted pages. Knowing what to cut can be difficult for an author who believes every word to be essential. There's tons of books on screenwriting; anything by Syd Field is a good place to start. Good luck.
You'd probably be well-served by posting this question in the Genre-Based, Screenplay forum, too.