Robert Downey Jr. Paul Avery
Jake Gyllenhaal Robert Graysmith
Mark Ruffalo David Toschi
Dermot Mulroney Martin Lee
Brian Cox Melvin Belli
Chloë Sevigny Melanie
Jimmi Simpson Mike Mageau
David Fincher Director
June Diane Raphael Mrs. Toschi
Clea DuVall Linda del Buono
George Drakoulias Music Supervisor
Elias Koteas Jack Mulanax
Anthony Edwards William Armstrong
Michael Rose Navy Project Leader
Adam Goldberg Duffy Jennings
John Carroll Lynch Arthur Leigh Allen
John Getz Templeton Peck
Candy Clark Carol Fisher
Donal Logue Ken Narlow
Jason Wiles Lab Tech Dagitz
Ione Skye Kathleen Johns (uncredited)
Paul Schulze Sandy Panzarella
James Vanderbilt Screenplay, Producer
Karina Logue Woman
Barry Livingston Copy Editor #3
Zach Grenier Mel Nicolai
James LeGros George Bawart
Charles Fleischer Bob Vaughn
Thomas Kopache Copy Editor #1
Tom Verica Jim Dunbar
Matt Winston John Allen
Philip Baker Hall Sherwood Morrill
John Lacy Zodiac 4
John Ennis Terry Pascoe
Lee Norris Young Mike Mageau
Adam Trese Detective #1
David Lee Smith Father
Stanley B. Herman Staff Editor
Bob Stephenson Zodiac 3
Jeff Daniel Phillips Informant
Joel Bissonnette Inspector Kracke
Angus Wall Editor
Richmond Arquette Zodiac 1 / Zodiac 2
John Terry Charles Thieriot
Ren Klyce Supervising Sound Editor, Sound Designer
Pell James Cecilia Shepard
Louis Phillips Executive Producer
Judith Drake Florence Douglas
Mickey Giacomazzi Stunt Coordinator
Brett Rickaby Detective Roy
Christopher John Fields Copy Editor #4
John Mahon Riverside Captain
Ciara Hughes Darlene Ferrin
Jules Bruff Catherine Allen
Callie Thompson Child Witness (uncredited)
J. Patrick McCormack Police Commissioner
JD Cullum Director
Randall Poster Music Supervisor
Bryan Lawson Music Editor
Bradley J. Fischer Producer
Anna Katarina Society Woman
Cazimir Milostan Internal Affairs Agent #2
Brad Carr Camera Man (uncredited)
Patrick Scott Lewis Bryan Hartnell
Drew Kunin Sound mixer
Richard Quinn Dialogue Editor
Michael Semanick Sound Re-Recording Mixer
James Joseph O'Neil DOD Project Leader
Mike Medavoy Producer
Betty Murphy Informant
Daniel M. Stillman Unit Production Manager
Cassius M. Willis Uniform Cop