Robert Downey Jr. Paul Avery
Jake Gyllenhaal Robert Graysmith
Dermot Mulroney Martin Lee
Mark Ruffalo David Toschi
Brian Cox Melvin Belli
David Fincher Director
Chloë Sevigny Melanie
Jimmi Simpson Mike Mageau
Adam Goldberg Duffy Jennings
Anthony Edwards William Armstrong
Donal Logue Ken Narlow
Clea DuVall Linda del Buono
Elias Koteas Jack Mulanax
John Carroll Lynch Arthur Leigh Allen
James Le Gros George Bawart
Paul Schulze Sandy Panzarella
June Diane Raphael Mrs. Toschi
Ione Skye Kathleen Johns (uncredited)
John Getz Templeton Peck
Philip Baker Hall Sherwood Morrill
Karina Logue Woman
Richmond Arquette Zodiac 1 / Zodiac 2
Tom Verica Jim Dunbar
Thomas Kopache Copy Editor #1
Candy Clark Carol Fisher
Zach Grenier Mel Nicolai
James Vanderbilt Screenplay, Producer
Lee Norris Young Mike Mageau
Mickey Giacomazzi Stunt Coordinator
Matt Winston John Allen
Jason Wiles Lab Tech Dagitz
John Terry Charles Thieriot
Bob Stephenson Zodiac 3
Charles Fleischer Bob Vaughn
Adam Trese Detective #1
J. Patrick McCormack Police Commissioner
David Lee Smith Father
Jeff Daniel Phillips Informant
Anna Katarina Society Woman
Barry Livingston Copy Editor #3
John Ennis Terry Pascoe
Judith Drake Florence Douglas
Cooper Thornton Internal Affairs Agent #1
Pell James Cecilia Shepard
Christopher John Fields Copy Editor #4
George Drakoulias Music Supervisor
Joel Bissonnette Inspector Kracke
Bud Davis Patrolman (uncredited)
JD Cullum Director
Richard Hymns Supervising Sound Editor
David Parker Sound Re-Recording Mixer
Ceán Chaffin Producer
Ciara Hughes Darlene Ferrin
Michael Semanick Sound Re-Recording Mixer
Charles Schneider Cabbie / Paul Stine
Angus Wall Editor
Ren Klyce Supervising Sound Editor, Sound Designer
John Hemphill Donald Cheney
Brett Rickaby Detective Roy
Daniel M. Stillman Unit Production Manager
Randall Poster Music Supervisor
Laray Mayfield Casting
Harris Savides Director of Photography
Stanley B. Herman Staff Editor
Ted Garcia Anchorman #1
James Carraway Shorty
Malcolm Fife Dialogue Editor
Addison Teague Sound Effects Editor
Scotty Richards Stunt Coordinator
Patrick Scott Lewis Bryan Hartnell
Betty Murphy Informant
Donald Graham Burt Production Design