- Paul Newman
- General Leslie R. Groves
- Dwight Schultz
- J. Robert Oppenheimer
- Bonnie Bedelia
- Kitty Oppenheimer
- John Cusack
- Michael Merriman
- Laura Dern
- Kathleen Robinson
- Ron Frazier
- Peer de Silva
- John C. McGinley
- Richard Schoenfield
- Natasha Richardson
- Jean Tatlock
- Ron Vawter
- Jamie Latrobe
- Michael Brockman
- William 'Deke' Parsons
- Del Close
- Dr. Kenneth Whiteside
- John Considine
- Robert Tuckson
- Allan Corduner
- Franz Goethe
- Joe D'Angerio
- Seth Neddermeyer
- Jon DeVries
- Johnny Mount
- James Eckhouse
- Robert Harper
- Todd Field
- Robert Wilson
- Mary Pat Gleason
- Dora Welsh
- Clark Gregg
- Douglas Panton
- Péter Halász
- George Kistiakowsky
- Gerald Hiken
- Leo Szilard
- Arthur Holden
- Oakridge Doctor
- Ed Lauter
- Whitney Ashbridge
- Donald MacKechnie
- James Tuck
- Madison Mason
- Boris Pash
- Christoph Pieczynski
- Otto Frisch
- Don Pugsley
- Bronston
- Logan Ramsey
- Brehon Somervell
- Fred Thompson
- Melrose Hayden Barry
- Jim True-Frost
- Donald Hornig
- Barry Yourgrau
- Edward Teller
- Marek Alboszta
- Scientist
- Steven Baigelman
- Dr. Avenell
- Frank Benettieri Jr.
- Messenger
- Roger Cubicciotti
- Frank Oppenheimer
- Franco Cutietta
- Enrico Fermi
- Robert Peter Gale
- Dr. Louis Hempelman
- Wesley Harrison
- Peter Oppenheimer
- Brent Harrison
- Peter Oppenheimer
- Tom McFarlane
- Scientist
- David C. Parnes
- Raincoat Man
- Allen Poirson
- Harold McDonald
- David Politzer
- Robert Serber
- Bill Rubenstein
- Times Reporter
- Ken Strausbaugh
- Observation Officer
- Walter Sullivan
- Henry Stimson
- Brian Wandell
- Dennis Talmudge
- John Williams
- Mack Stoddard
- Walker Edmiston
- (voice)
- Matthew Faison
- (voice)
- Stan Jones
- (voice) (as Stanley Jones)
- Alan Oppenheimer
- (voice)
- David Brainard
- Samuel Allison
- William J. Bruno
- Scientist
- Marla Feller
- Scientist
- David Smith
- Scientist
- Kevin Tighe
- Committee Member
