Some of the planned New York locations became unsuitable and it was brought to Leone's attention that Montreal, which was the capital of Prohibition, had more buildings and details of the 1930s than New York.  During a scouting trip to Montreal in October 1975, Leone declared that the cast of the movie would include Gerard Depardieu as Noodles and Richard Dreyfuss as Max.  Jean Gabin would play the older Max and maybe James Cagney the older Noodles. And there could be guest appearances by some of the great stars of the golden age, such as George Raft, James Stewart, Henry Fonda and Glenn Ford.  There would also be an important part for French-Canadian singer actor Robert Charlesbois. And part of the story would be set in Canada.  Cagney was said to be flattered by the proposition but had trembling hands and Richard Dreyfuss did not feel it was the right moment for him to take on the part of Max.   Gerard Depardieu very much wanted a role in the movie and was prepared to learn English with the correct accent if required.

De Niro had been approached much earlier, during the making of Godfather Part 11 (1974) and Leone offered him the choice of playing either lead.  It became apparent that De Niro would be able to play Noodles as an adult and an old man and De Niro was actively involved with choosing the remaining cast members.

Pre-production casting was extensive - 3000 actors were interviewed for 110 speaking roles and 500 auditions were videotaped.  Actors for the role of Max included Jon Voight, William Hurt, Joe Pesci, Harvey Keitel and John Malkovich.  Liza Minelli and Geena Davis auditioned for the part of Deborah.  It's reported that Clint Eastwood turned down the part of Jimmy O'Donnell.

Final cast & crew:
Robert De Niro David 'Noodles' Aaronson
James Woods Maximilian 'Max' Bercovicz
Elizabeth McGovern Deborah Gelly
Tuesday Weld Carol
Treat Williams James Conway O'Donnell (Jimmy)
James Hayden Patrick 'Patsy' Goldberg
Joe Pesci Frankie Minaldi
Larry Rapp 'Fat' Moe Gelly
Danny Aiello Police Chief Vincent Aiello
William Forsythe Philip 'Cockeye' Stein
Burt Young Joe
Darlanne Fluegel Eve
Dutch Miller Van Linden
Robert Harper Sharkey
Richard Bright Chicken Joe
Gerard Murphy Crowning
Amy Ryder Peggy
Olga Karlatos Woman in the Puppet Theatre
Mario Brega Mandy
Ray Dittrich Trigger
Frank Gio Beefy
Karen Shallo Lucy Aiello
Angelo Florio Willie The Ape
Scott Tiler Young Noodles
Rusty Jacobs Young Max/David Bailey
Brian Bloom Young Patsy
Adrian Curran Young Cockeye
Mike Monetti Young 'Fat' Moe
Noah Moazezi Dominic
James Russo Bugsy
Frankie Caserta Bugsy's Gang Member #1
Joey Marzella Bugsy's Gang Member #2
Clem Caserta Al Capuano
Frank Sisto Fred Capuano
Jerry Strivelli Johnny Capuano
Julie Cohen Young Peggy
Marvin Scott Marvin Brentley
Mike Gendel Irving Gold
Paul Herman Monkey
Ann Neville Girl in Coffin
Joey Faye Adorable Old Man
Linda Ipanema Nurse Thompson
Tandy Cronyn Reporter #1
Richard Zobel Reporter #2
Baxter Harris Reporter #3
Arnon Milchan Limousine Chauffeur
Bruno Iannone Thug
Marty Licata Cemetery Caretaker
Marcia Jean Kurtz Max's Mother
Estelle Harris Peggy's Mother
Richard Foronjy Officer 'Fartface' Whitey
Gerritt Debeer Drunk
Jennifer Connelly Young Deborah
Alexander Godfrey Newstand Proprietor
Cliff Cudney Mounted Policeman #1
Paul Farentino Mounted Policeman #2
Bruce Bahrenburg Sgt. P. Halloran
Mort Freeman Street Singer
Sandra Solberg Friend of Young Deborah
Jay Zeely Foreman
Massimo Liti Macro
Salvatore Billa One of Beefy's Thugs (uncredited)
Matteo Cafiso Boy in Park (uncredited)
Nelson Camp Newspaper Salesman (uncredited)
Scott Coffey Teenager (uncredited)
Francesca Leone David Bailey's Girlfriend (uncredited)
Chuck Low Deborah Gelly's father (uncredited)
Produced by Claudio Mancini
Executive Producer Arnon Milchan
Original Music by Ennio Morricone
Cinematography by Tonino Delli Colli
Film Editing by Nino Baragli
Casting by Cis Corman Joy Todd
Production & Art Design by Carlo Simi
Costume Design by Gabriella Pescucci