Director Sidney Lumet had the actors all stay in the same room for several hours on end and do their lines over and over without filming them. This was to give them a real taste of what it would be like to be cooped up in a room with the same people.
At the beginning of the film, the cameras are all positioned above eye level and mounted with wide-angle lenses to give the appearance of greater distance between the subjects. As the film progresses, the cameras slip down to eye level. By the end of the film, nearly all of it is shot below eye level, in close-up and with telephoto lenses to increase the encroaching sense of claustrophobia.
Because the film failed to make a profit, Henry Fonda never received his deferred salary. Despite this setback, he always regarded this film as one of the three best he ever made. The others being The Grapes of Wrath (1940) and The Ox-Bow Incident (1942).
Because the painstaking rehearsals for the film lasted an exhausting two weeks, filming had to be completed in an unprecedented 21 days.
Henry Fonda disliked watching himself on film, so he did not view the whole film in the screening room. However, before he walked out, he said quietly to director Sidney Lumet, "Sidney, it's magnificent."