The murder victims' costumes were meticulously recreated from forensic evidence that was lent to the production.
When Mark Ruffalo met David Toschi, the investigator he plays in the film, he was very impressed to learn that Toschi had perfect recall of every detail of every case.
Because he wanted the film to be as accurate as possible, David Fincher decided not to depict any of the alleged Zodiac murders for which there were no surviving victims or witnesses.
The producers hired a private investigator to track down the real-life Zodiac survivor, Michael Renault Mageau.
The shooting script was 200 pages long. To prevent any problems with length that such a long script might cause, director David Fincher decided to ask his cast members to speak faster.
Bryan C. Hartnell: the real-life survivor of the Lake Berryessa attack appears as an extra alongside his wife in the background of the scene where Marty Lee suggests to David Toschi that he take some time off work.
Ione Skye: as Kathleen Johns, the female driver who was abducted by a man (whom she would later identify as the Zodiac based on the composite sketch for the man wanted for the killing of Paul Stine) who threatens to kill her and her baby.