Despite the advice given in its last line of dialogue, the one thing you can't do with Chinatown is forget it. Regarded by nearly everyone—from the American Film Institute to IMDb users—as one of the best movies ever made, Roman Polanski's masterpiece is a modern film noir with a labyrinthine plot and deeply sinister undertones, with top-of-their-game performances by Jack Nicholson, Faye Dunaway, John Huston, and, well, just about everyone in it. (Don't forget Rance Howard as Irate Farmer!) Here are a dozen facts about Chinatown, all as plain as the sliced nose on Jake's face. 

1. IT WOULDN'T EXIST IF ROBERT TOWNE HADN'T BEEN FRIENDS WITH JACK NICHOLSON. 

The screenwriter and the actor were good friends, even roommates at one point, and they'd studied acting together. Towne has said repeatedly that he wrote the lead role specifically for Nicholson: "I could not have written that character without knowing Jack." Furthermore, it was while visiting Nicholson in Oregon, where he was directing Drive, He Said, that Towne started reading Raymond Chandler detective novels and a book about the history of California water rights, all of which led to Chinatown

2. THE SCREENPLAY DIDN'T HAVE ANY SCENES ACTUALLY SET IN CHINATOWN.

Chinatown is a symbol in Towne's screenplay, representing "the futility of good intentions" (as he said in a DVD interview). And in his original screenplay, it was just a metaphor, with none of the action taking place there. Director Roman Polanski suggested it would be more satisfying if the film's climax took us to the very place J.J. Gittes never wanted to return to, literally as well as symbolically. 

3. POLANSKI CONVINCED TOWNE TO CHANGE THE ENDING, TOO.

In the original version, Evelyn Mulwray fatally shoots her father, but since she refuses to explain her reasons, she's destined for life in prison. "Not a happy ending," Towne said, "but a more complex ending." Polanski wanted to go even darker: Evelyn takes a shot at Dad but only wounds him, while she herself ends up dead, leaving poor Katherine in the hands of the nasty old man. Towne thought that was too melodramatic but ultimately ceded the battle to Polanski. He eventually acknowledged that Polanski's version was better. 

4. ROBERT TOWNE TURNED DOWN $175,000 TO ADAPT THE GREAT GATSBY AND TOOK $25,000 TO WRITE CHINATOWN INSTEAD.

Uber-producer and Hollywood playboy Robert Evans liked the (mostly uncredited) script doctoring Towne had done on Bonnie and Clyde and The Godfather, and offered him what was then a very large sum of money to adapt F. Scott Fitzgerald's The Great Gatsby. Towne declined for an understandable reason: "I didn't want to be the unknown Hollywood writer who f***** up a literary classic." (That honor eventually went to Francis Ford Coppola.) Instead, Towne said, he wanted to develop his Chinatown idea. For this Evans offered him $25,000. (Don't worry about Towne, though. When the film actually went into production, he got another $250,000, plus five percent of the gross.) 

5. POLANSKI ALMOST DIDN'T DO HIS CAMEO BECAUSE HE DIDN'T WANT TO CUT HIS HAIR. 

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The director cast himself in the role of the bow-tied thug who slices Gittes' nose. But Polanski had long hair at the time, which was wrong for the character, and he was reluctant to cut it—reluctant almost to the point of backing out and casting someone else. Finally he relented, but he got the haircut at the last possible minute, right there on the set before shooting the scene. 

6. NOAH CROSS CONSISTENTLY MISPRONOUNCING GITTES' NAME WAS A MISTAKE, NOT A CHOICE.

It certainly suits the character of a rich, evil man not to care whether he gets some dumb detective's name right. But in truth, the reason Cross keeps calling him "Gits" instead of "Git-is" is that the actor, John Huston, couldn't get it right. Polanski had Nicholson add a line trying to correct him, and after that just let it go. 

7. IT WAS INTENDED AS THE FIRST PART IN A TRILOGY—BUT NO, THE ABORTED THIRD PART DID NOT BECOME WHO FRAMED ROGER RABBIT.

Part two was The Two Jakes, which Towne wrote and Nicholson directed in 1990. Part three, which never did get written, was to have been called Gittes vs. Gittes (not Cloverleaf, as the legend goes), about the detective's divorce, and would have involved corruption relating to L.A.'s land ownership and transportation system. An urban legend has sprung up that this third plot was the basis for Who Framed Roger Rabbit, which does involve a private detective in 1940s L.A. uncovering a scheme to dismantle public transportation and buy up land for a freeway system. But Roger Rabbit came out in 1988, two years before the failure of The Two Jakes meant Gittes vs. Gittes wasn't going to happen. Moreover, Roger Rabbit was adapted, albeit loosely, from a 1981 novel (Who Censored Roger Rabbit, in which they were comic strip characters, not cartoons). Who Framed Roger Rabbit was clearly inspired by Chinatown, as were many crime films set in old-timey L.A., and it bore some coincidental similarities to Towne's unproduced idea. But there was no official connection. 

8. THE MUSICAL SCORE WAS A LAST-MINUTE REPLACEMENT, WRITTEN IN NINE DAYS.

Polanski's go-to composer, Krzysztof Komeda, had died a few years earlier, so Polanski ended up hiring a man named Phillip Lambro to do the honors for Chinatown. After a test screening, however, Polanski's friend Bronislaw Kaper—an old-school Hollywood guy and a composer in his own right—told him: "Great movie, but you have to change the music." (Producer Robert Evans was already leaning in that direction.) Jerry Goldsmith was hired to write a new score, and did it in nine days. His rush job was rewarded with an Oscar nomination, one of seven he received in the 1970s alone. (Some of Lambro's score survives in the Chinatown trailer.)

9. WE DON'T KNOW ANYTHING THAT J.J. GITTES DOESN'T KNOW. 

This is the sort of detail that's either "well, duh" obvious, or that blows your mind a little when you realize it. The film is entirely from Gittes' point of view: he's in every scene, and there's no information that we learn before he does. When he gets a phone call, we hear the voice but don’t see the person at the other end. When he gets knocked unconscious in the orange grove, the movie fades with him, fading back in when he wakes up. To emphasize the point that we're seeing everything from Gittes' perspective, Polanski often put the camera behind Nicholson, so we see his back and shoulders. Watch for it.  

10. TOWNE'S ACCLAIMED SCREENPLAY OWES A LOT TO POLANSKI.

Towne won an Oscar for his screenplay, the only one of 11 nominations that came through for Chinatown. (It was the year of The Godfather: Part II.) The script is used in screenwriting courses and is often held up as an example of a perfect screenplay. But Towne's first version was 180 pages long (which would have made a three-hour movie), and hopelessly complicated. "It would have been a mess" if they'd filmed that version, Towne later said. It was when Polanski came to L.A. in the spring of 1973 and spent eight weeks painstakingly rewriting the script with Towne that it really came together. They worked on it every day and, by both men's admission, fought every day, about everything. Polanski crafted the story structure, and Towne would write the dialogue. Towne's ex-wife (who admittedly had an ax to grind) later told a biographer, "Roman could have easily asked for a [writing] credit on Chinatown and he would have gotten it. It wasn't just the ending. Roman simply took it over, structured the whole piece." 

11. THE CUCKOLDED HUSBAND WE MEET IN THE FIRST SCENE USED TO HAVE MORE SINISTER PLANS.

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Poor Curly, played by Burt Young (soon to be Paulie in Rocky), has hired Gittes to find out if his wife is cheating on him, which she is. A view of the scandalous photos is the first thing we see after the opening credits. Originally, the scene had an exchange where Curly tells Gittes he's going to kill the unfaithful woman, and Gittes tells him he's not rich enough to get away with murder. (That's why they're talking about Curly paying his bill as they come out of the office, and why Gittes says, "I only brought it up"—Curly's financial situation—"to illustrate a point.") Towne later regretted removing this part of the scene. "That exchange I miss probably as much as any in the movie," he said in 1999. "Because it really foreshadows [the] 'You've got to be rich to kill somebody and get away with it' [theme]. He's really foreshadowing the whole movie." 

12. THERE'S A RECURRING VISUAL MOTIF THAT SHOWS UP OVER AND OVER—AND IT'S THERE ACCIDENTALLY.

Chinatown frequently shows us images of two things that are identical, except that one is flawed: Two pocket watches side by side, one broken. A pair of eyeglasses, one lens cracked. Gittes' nostrils, one sliced. Gittes smashes one taillight on Evelyn's car. He loses one shoe in the reservoir. Evelyn has a flaw in one of her irises. Katherine looks like a duplicate of Evelyn, but is the product of incest. The list goes on. But when Towne is asked about this on the DVD commentary, he says it was totally unintentional; he and Polanski never discussed using such images as a recurring theme. Whatever meaning we may ascribe to the symbolism, the filmmakers didn't put it there on purpose. 

Additional Sources:
DVD/Blu-ray commentary and special features
Easy Riders, Raging Bulls, by Peter Biskind
American Film Institute