In order to write Chinatown, Robert Towne turned down $175,000.
You read that correctly. Producer Robert Evans offered him that sum in order to adapt The Great Gatsby, but Towne didn't think he could do justice to a literary classic of that caliber. So, he asked for $25,000 instead in order to work on his own project.
And he wrote Chinatown.
In retrospect, it was the right move. Not only is Chinatown considered one of the best screenplays ever written, but Francis Ford Coppola ended up writing The Great Gatsby and it received decidedly mixed reviews. (Source)
Towne went ahead with his script about water theft, political corruption, and incest. In an interview, he said that he had Jack Nicholson in mind for the lead from the very beginning, and that, originally, he gave the movie a happy ending.
He thought that would be, in a way, more daring—since unhappy endings were the fashion. After going into the darkness with the big incest revelation, it seemed like you were bucking this cynical trend if you let Evelyn kill Noah and survive at the end. But, when Roman Polanski became the director, he insisted on changing it, and ultimately won the debate. (Source)
Towne didn't just write complicated mysteries with incest revelations at the end, though. He also wrote the screenplay for Hal Ashby's classic comedy Shampoo (1975), which was about a hairdresser who's also a ladies man (played by Warren Beatty), and the first two Mission Impossible movies.