As preparation for his traumatic scene in the interrogation room, Leland Orser would hyperventilate, so that his body would be overly saturated with oxygen. 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.
Brad Pitt fell while filming the scene in which Mills chases John Doe in the rain. Pitt's arm went through a car windshield, requiring surgery. The accident was worked into the script. Coincidentally, the original script called for Detective Mills to be injured during the sequence.
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).