Robert Downey Jr. Paul Avery
Jake Gyllenhaal Robert Graysmith
Mark Ruffalo David Toschi
Adam Goldberg Duffy Jennings
David Fincher Director
Jimmi Simpson Mike Mageau
Brian Cox Melvin Belli
Chloë Sevigny Melanie
Dermot Mulroney Martin Lee
Elias Koteas Jack Mulanax
Donal Logue Ken Narlow
Clea DuVall Linda del Buono
Paul Schulze Sandy Panzarella
John Carroll Lynch Arthur Leigh Allen
James Le Gros George Bawart
Anthony Edwards William Armstrong
Ione Skye Kathleen Johns (uncredited)
Zach Grenier Mel Nicolai
Matt Winston John Allen
Jason Wiles Lab Tech Dagitz
Tom Verica Jim Dunbar
June Diane Raphael Mrs. Toschi
Candy Clark Carol Fisher
Philip Baker Hall Sherwood Morrill
John Getz Templeton Peck
Richmond Arquette Zodiac 1 / Zodiac 2
John Terry Charles Thieriot
Jeff Daniel Phillips Informant
Lee Norris Young Mike Mageau
Karina Logue Woman
George Drakoulias Music Supervisor
Bob Stephenson Zodiac 3
Thomas Kopache Copy Editor #1
James Vanderbilt Screenplay, Producer
Judith Drake Florence Douglas
Barry Livingston Copy Editor #3
John Ennis Terry Pascoe
Pell James Cecilia Shepard
Danielle McKee Prisoner (uncredited)
Charles Fleischer Bob Vaughn
John Lacy Zodiac 4
Christopher John Fields Copy Editor #4
Michael Rose Navy Project Leader
Cazimir Milostan Internal Affairs Agent #2
Ceán Chaffin Producer
Trish Almeida Key Hair Stylist
Cooper Thornton Internal Affairs Agent #1
Donald Graham Burt Production Design
David Lee Smith Father
Brett Rickaby Detective Roy
J. Patrick McCormack Police Commissioner
Michael Semanick Sound Re-Recording Mixer
Laray Mayfield Casting
Adam Trese Detective #1
Edson Williams Visual Effects Supervisor
Randall Poster Music Supervisor
Bradley J. Fischer Producer
Cassius M. Willis Uniform Cop
Addison Teague Sound Effects Editor
Mickey Giacomazzi Stunt Coordinator
Shane Woodson Informant
John Hemphill Donald Cheney
JD Cullum Director
Bud Davis Patrolman (uncredited)
Joel Bissonnette Inspector Kracke
Angus Wall Editor
Brad Carr Camera Man (uncredited)
Malcolm Fife Dialogue Editor
David Parker Sound Re-Recording Mixer
Betty Murphy Informant
Richard Hymns Supervising Sound Editor
Ed Setrakian Al Hyman