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

Ken Pogue

Dwight D. Eisenhower

Gary Reineke

Groves's Commanding Officer

Alan Scarfe

John Seitz

Ralph A. Bard

Tony Shalhoub

Enrico Fermi

Ron White

Colonel Nichols

Patrick Breen

Richard Feynman

Vlasta Vrana

Hans Bethe

Lorne Brass

Klaus Fuchs

Michael Sinelnikoff

Lord Rutherford

Kliment Denchev

Michael Polanyi

Timothy Webber

Colonel Lansdale

Harry Standjofski

Robert Wilson

Lia Sargent

Mrs. Wisnovsky

David Gow

Jack Wisnovsky

Isabelle Mejias

Mrs. Trowbridge

Jonathan Wise

Robert Serber

George R. Robertson

Ed Condon

Dee McCafferty

William 'Deke' Parsons

Tom Butler

Captain DeSilva

David Ferry

Army Major

Michael J. Reynolds

Kenneth Bainbridge

Scott Thomson

Chemist

Roland Hewgill

Army General

Arthur Holden

Scientist (uncredited)

Romano Orzari

Leo Szilard's Prime Scientist (uncredited)