The relationship between cinema and literature is a complex give and take. One thing I've learned from my project is that the two forms of media are inextricably linked. So it comes as no surprise that novelists will sometimes write film scripts. What does come as a surprise are the specific examples I've dug up.
#1 The Big Sleep
The Big Sleep is one of the most famous film noir movies ever made. Starring Humphrey Bogart and Lauren Bacall, the 1946 film is based on the 1939 novel of the same name by Raymond Chandler. Set in Los Angeles, California, The Big Sleep (the book and movie) is largely responsible for defining the the hard-boiled crime novel. So, who did the studios get to write it? Chandler? Dashiell Hammett? James Cain?
...Written by William Faulkner
William Faulkner, known and acclaimed for his novels about the post-Civil War South. While the subject matter may seem a surprising departure from Faulkner's usual area of interest, the complex (perhaps even convoluted) manner of storytelling is right up Faulkner's alley
#2 Pride and Prejudice
The 1940 version of Jane Austen's 1813 novel of the same name, starring Greer Garson and Laurence Olivier is considered a masterpiece. At the same time a love story and a criticism of the society she lived in, Austen's novel could draw any number of screenwriters.
...Written by Aldous Huxley
Aldous Huxley, best known for Brave New World, seems an unlikely, if appropriate, choice for screenwriter. The exaggeration stylistically inherent in romantic era literature leads people to forget that, like Huxley, Austen was a satirist. And while people think of Huxley in terms of futurism, other novels of his (like Crome Yellow), are contemporary critiques on British society. Huxley also wrote the screenplay for the 1943 version of Charlotte Brontë's Jane Eyre, starring Orson Welles and Joan Fontaine.
#3 Moby Dick
There have only been a couple film versions of Melville's 1851 novel. In fact, putting aside the Made-for-TV movies, only three film version have been theatrically released: the 1926 silent film, The Sea Beast, a 1930 version that is only very loosely connected to the novel, and the 1956 version starring Gregory Peck as Ahab. Who would you get to write something like this? Hemingway, maybe?
...Written by Ray Bradbury
Ray Bradbury, best known as a science fiction/fantasy writer, and author of works including Fahrenheit 451 and Something Wicked This Way Comes, is not what anyone would expect for a project like this. But hey, you can't argue with results.
#4 Sex and the Single Girl
In 1962, Helen Gurley Brown published the non-fiction advice book, Sex and the Single Girl, advocating sexual freedom for women. The 1964 film is a farce centering around Dr. Helen Gurley Brown (the fictional version having apparently earned a PhD.) played by Natalie Wood, also starring Tony Curtis, Henry Fonda, and Lauren Bacall. So, a raunchy sex comedy (by 1960's standards). Maybe Jacqueline Susann? Sidney Sheldon won an Oscar for this kind of thing (1947's The Bachelor and the Bobby-Soxer).
...Written by Joseph Heller
Joseph Heller!? The man is famous for his gallows humor! He wrote Catch-22, one of the finest novels of his generation. That said, he didn't seem to get the same critical respect for the films he worked on. Sex and the Single Girl was a financially successful critical failure. As was 1967's Peter Seller's spy spoof, Casino Royale (for which Heller remained uncredited). His only other screenplay, 1970's Dirty Dingus Magee, seems to have not really gained any traction.
#5 You Only Live Twice
Ninjas! Space Kidnappings! Evil Cat-Guy! It's got the crazy over the top action of a Roger Moore Bond Movie, but the suavity of Sean Connery! While a pretty serious departure from Ian Fleming's novel, it must have been written by some action/espionage writer, right?
...Written by Roald Dahl
The man who brought the world Charlie and the Chocolate Factory, Matilda, The BFG, and James and the Giant Peach, also brought us an awesome spy/action/adventure movie. This isn't only time Dahl adapted one of Fleming's novels. Ian Fleming wrote a novel titled Chitty-Chitty-Bang-Bang: The Magical Car. Roald Dahl added the Child Catcher, a villain who kidnaps the protagonists. Just to reiterate, Ian Fleming wrote a novel about a magical flying car, and Roald Dahl added a diabolical villain. Something seems backwards here.
#6 Superman and Superman 2
The two best Superman movies. Ah, Superman. A character so morally unambiguous that a gritty reboot just can't cut it. These two films have action, adventure, romance, and are a lot of fun.
...Written by Mario Puzo
Really? That Mario Puzo? Someone, at some point actually said, "Hey, you know who would be great to right this light-hearted superhero movie? The guy who wrote The Godfather." I'm not arguing with the results, I'm just a bit surprised at how random this seems.