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The producers intended that Kevin Spacey should receive top billing at the start of the movie, but he insisted that his name not appear in the opening credits, so as to surprise the audience with the identity of the killer. To compensate, he is listed twice in the closing credits: once before the credits start rolling, and once in the rolling credits in order of appearance. Another advantage from Spacey's point of view, as he saw it, was that he was excluded from the film's marketing during its release, meaning he didn't have to make any public appearances or do any interviews.
As preparation for his traumatic scene in the interrogation room, Leland Orser would breathe in and out rapidly, so that his body would be overly saturated with oxygen, giving him the ability to hyperventilate. He also did not sleep for a few days, in order to achieve his character's disoriented look.
New Line Cinema executives originally balked at the film's ending, but Brad Pitt refused to make the film if the ending was changed.
When Mills and Somerset go to Wild Bill's Leather Store looking for information about John Doe, the clerk says that Doe "had a limp". Behind them, outside the store, a man limping is shown watching the detectives.
David Fincher told Kevin Spacey and Brad Pitt "this is not going to be the movie that you're remembered for, but it may be a movie you're incredibly proud of."
Charles S. Dutton: The cop who keeps the press out of the "greed" crime scene. Dutton had a substantial role in director David Fincher's previous movie Alien 3 (1992).