I know dozens of novels get adapted into motion pictures each year and there are plenty of great books I've read that I've though "That would make a really good movie," but I've never actually adapted one.

Here's my question: when a writer adapts of novel into a screenplay, on an average (this is actually a question best suited for someone who has read a lot of screenplays adapted from novels), how much of the written text is turned into screenplay. For instance, how much dialogue is FROM the novel and how much is original. Sadly the only films I've seen recently that come from books I've read are the Harry Potter films and I read those books so long ago I could never tell what was which.

I'm asking this because if I ever adapt a novel (I know I would have to get permission if I ever wanted to get it sold) how much dialogue should be taken from the actual book and how much of it original. I'm just wondering because I would want to write a good script, and I wouldn't want people thinking "Oh, he just cheated and used everything the original author said." What do the professional adapters use?

My adviice is only keep dialogue that actually affects the plot or proves a point to the reader of the novel.  For instance, in The Da Vinci code, Akiva Goldsman has a lot of the dialogue his own in the dead scenes of the film, but when it came time to describe the anagrams and cryptography stuff, he nearly copied Dan Brown's work right out of the book.

Basically if it's important dialogue, I think it would behoove you to stay as close to the wording as possible.  But for simple expository scenes or dead scenes, I don't see anything wrong with adding a bit of your own flavor to it.  Hell you might even want to add a scene that isn't in the book.  

Remember it's all up to you.  You're the writer.  

Okay, thanks. I just want to know whether or not it would be frowned upon if I used a lot of the already written dialogue. The novel I'm considering adapting has brilliant dialogue, I don't see how I could improve upon it.

Any other opinions would be greatly appreciated.

One thing that I've read about adaptations is to boil down the novel to the main points (usually 10) and use those points as a starting place to tell the screen story. This is usually the 10 most key scenes in the novel. From those scenes, you orient the screenplay so that you hit those main points. The biggest thing to remember is that a book usually deals with internal conflict and psychology, while a movie deals with all things visual. Books and movies can tell the same story, but must tell them in different ways because they are different mediums. This is why people who love the book usually hate the movie.

Some books do lend themselves to being good movies, such as Harry Potter, which is a very visual series of books. You can usually "see" the stuff lift off the page with incredible ease, which is likely why they wanted to adapt it before Rowling even finished the series. The first two Harry Potter movies were very direct adaptations o the novels which very few differences. The last two are more likely taking the key scenes and building the script around them. Lord of the Rings is another almost direct adaptation, but it has quite a few differences that use the boil-down approach (most notably Frodo's abrupt departure as opposed to his throwing a big going away party after several years).

Movies also make significant changes for visual purposes. Internal dialogue is very effective in books, but usually doesn't translate well to screen. Internal dialogue in movies is done through voiceovers (ref: A Clockwork Orange). What is sometimes done in an instance like this is to add a character for the sole purpose of getting that internal dialogue into a discussion somehow. Pet Sematary used the character of Victor Pascow that was in the novel who died early on the story as a secondary character to which the lead could converse. He was essentially Louis Creed's conscience in the movie, but in the book, he was in a single scene.

Those are my thoughts on it and what I've read about it. To answer your specific question, unless absolutely necessary, there is no need to lift the dialogue directly from the book unless you just love it. Rarely is the dialogue verbatim just because it usually won't fit the needs of the screen story.

Thanks for all the information. You were all a big help.