Brian Dennehy
— Gen. Leslie Groves
David Strathairn
— J. Robert Oppenheimer
Michael Tucker
— Leo Szilard
Hume Cronyn
— James F. Byrnes
Richard Dysart
— President Harry S. Truman
Hal Holbrook
— Gen. George Marshall
Barnard Hughes
— Henry Stimson
John McMartin
— Dr. Arthur Compton
David Ogden Stiers
— Franklin D. Roosevelt
Anne Twomey
— Kitty Oppenheimer
Lawrence Dane
Ron Frazier
— Colonel Pash
Olek Krupa
— Edward Teller
Bernie McInerney
John Pielmeier
— Seth
Ken Pogue
— Dwight D. Eisenhower
Gary Reineke
— Groves's Commanding Officer
Alan Scarfe
John Seitz
— Ralph A. Bard
Tony Shalhoub
— Enrico Fermi
Ester Spitz
— Trude Weiss
Ron White
— Colonel Nichols
Stephan Balint
— Eugen Wigner
Peter Boretski
— Albert Einstein
Patrick Breen
— Richard Feynman
Vlasta Vrana
— Hans Bethe
George Popovich
— Samuel Goudsmit
Lorne Brass
— Klaus Fuchs
Michael Sinelnikoff
— Lord Rutherford
Graham Haley
— Haakon Chevalier