Jack Nicholson was genuinely nervous during the nose-cutting scene because the knife being used could actually have hurt him badly if not held correctly. In the end, Roman Polanski did the scene himself to get it right.
During the infamous scene where Evelyn reveals that her sister is also her daughter, the slaps that Jake gave her were apparently genuine. Faye Dunaway mentioned that the fake smacks weren't leaving their intended impact in the scene, so she told Nicholson to actually slap her.
Life Imitates Art: Shortly after the film was released, it was discovered that Jack Nicholson's "sister" was really his mother and his "parents" were really his grandparents, though his father was not his grandfather. His mother got pregnant as a teenager and his father ran off, so his grandparents hid the pregnancy.
Reality Subtext: The film's ending can only be attributed in part to Polanski's bitterness with the world. It actually reflects the reality that America had been awakened to by the civil rights movement, Watergate, and Vietnam, that the corruption of the rich and powerful was an epidemic.
Anjelica Huston, John's real life daughter, as Evelyn Mulwray. Squick.
Things were pretty awkward as it was, given that Jack Nicholson had just started dating Anjelica in real life, making the scenes where John's character asks "Mr. Gittes, do you sleep with my daughter?" just...uncomfortable.
Ali MacGraw and Jane Fonda were also up for the role before it went to Dunaway.
Chinatown was meant to be the first film in a trilogy, each starring Jake Gittes and revolving around corruption during the development of Los Angeles. Jack Nicholson never played another detective character, so that Gittes would remain his iconic PI. Unfortunately the sequel had trouble getting off the ground - thanks to Roman Polanski's rape of at least one underage girl and his subsequent flight to Europe to avoid prosecution - and when it finally materialized sixteen years later, its critical and commercial failure put a kibosh on any chances for the third film. Bizarrely enough, many elements of what would have been the third film turned up in Who Framed Roger Rabbit, notably the freeway arc.
Robert Towne originally wrote the screenplay with more of a Bittersweet Ending, in which Evelyn kills her father and goes to prison. Only after a long and bitter debate with Polanski was it changed - at the last minute - to the infamous Downer Ending. Towne later conceded that the film was better that way.