Jake Gyllenhaal as Robert Graysmith

Mark Ruffalo as David Toschi

Anthony Edwards as William Armstrong

Robert Downey Jr. as Paul Avery

Chloë Sevigny as Melanie

Elias Koteas as Jack Mulanax

John Carroll Lynch as Arthur Leigh Allen

Brian Cox as Melvin Belli

Dermot Mulroney as Martin Lee

Charles Fleischer as Bob Vaughn

Zach Grenier as Mel Nicolai

Philip Baker Hall as Sherwood Morrill

James Le Gros as George Bawart

Donal Logue as Ken Narlow

Richmond Arquette as Zodiac 1 / Zodiac 2

Bob Stephenson as Zodiac 3

John Lacy as Zodiac 4

Ed Setrakian as Al Hyman

John Getz as Templeton Peck

John Terry as Charles Thieriot

Candy Clark as Carol Fisher

June Diane Raphael as Mrs. Toschi

Ciara Hughes as Darlene Ferrin

Lee Norris as Young Mike Mageau

Patrick Scott Lewis as Bryan Hartnell

Pell James as Cecilia Shepard

David Lee Smith as Father

Jason Wiles as Lab Tech Dagitz

Charles Schneider as Cabbie / Paul Stine

James Carraway as Shorty

Tom Verica as Jim Dunbar

Jimmi Simpson as Mike Mageau

Doan Ly as Belli's Housekeeper

Karina Logue as Woman

Joel Bissonnette as Inspector Kracke

John Mahon as Riverside Captain

Matt Winston as John Allen

Jules Bruff as Catherine Allen

John Ennis as Terry Pascoe

J. Patrick McCormack as Police Commissioner

Adam Goldberg as Duffy Jennings

Clea DuVall as Linda del Buono

Paul Schulze as Sandy Panzarella

Adam Trese as Detective #1

Penny Wallace as Mulanax's Secretary

John Hemphill as Donald Cheney

Michel Francoeur as Man on Marquee

Thomas Kopache as Copy Editor #1

Barry Livingston as Copy Editor #3

Christopher John Fields as Copy Editor #4

Jack Samson as Young David Graysmith

Micah Sauers as David Graysmith

Zachary Sauers as Aaron Graysmith

Jessica Baltutis as Margot Graysmith

Peter Quartaroli as Patrolman Fouke

Geoff Callan as Patrolman Zelms

Michael Hungerford as Truck Driver

Anna Katarina as Society Woman

E.P. McKnight as Informant (Hall of Justice A)

Betty Murphy as Informant

Jeff Daniel Phillips as Informant

Shane Woodson as Informant

Cookie Crawford as Informant #8

Cassius M. Willis as Uniform Cop

Judith Drake as Florence Douglas

Phoebe Holston as Prison Guard

Marty Lodge as Pinole Foreman

Cooper Thornton as Internal Affairs Agent #1

Cazimir Milostan as Internal Affairs Agent #2

Charlotte Ferguson as Kathleen Johns' Baby

Phoebe Ferguson as Kathleen Johns' Baby

Kacey Malmsten as Kathleen Johns' Baby

Karly Malmsten as Kathleen Johns' Baby

JD Cullum as Director

Stanley B. Herman as Staff Editor

Roy Lee Jones as Distraught African American Male

Michael Rose as Navy Project Leader

James Joseph O'Neil as DOD Project Leader

Rod Damer as FBI Codebreaker

John Sarno as Donald Harden

Gloria Grant as Bettye Harden

Brett Rickaby as Detective Roy

M.F. Bernier as Detective #2

Carter Evans as Newscaster #2

Bill Seward as TV News Anchor

Dave Nemeth as Newscaster #4

Ted Garcia as Anchorman #1

Greg Wolf as Interviewer #1

Barry LeBrock as Reporter

Ione Skye as Kathleen Johns (uncredited)

David Winston Barge as Vallejo Desk Ofiicer (uncredited)

Hayati Akbas as Hardware Store Customer (uncredited)

Brad Carr as Camera Man (uncredited)

Mitchell Fink as Reporter (uncredited)

Erica Ford as Traveler (uncredited)

Tish Hicks as Informant (uncredited)

Danielle McKee as Prisoner (uncredited)

Derris Nile as Office Worker (uncredited)

Carmen Plumb as Janie (uncredited)

Callie Thompson as Child Witness (uncredited)

Bud Davis as Patrolman (uncredited)