Barbara's actual response to the guard advising her to "take a deep breath, it's easier" was supposedly "how _the hell_ would you know". Apparently it had to be cleaned up for the 1958 audience, which is ironic given the rather graphic nature of the scene.
Wise asked the warden of San Quentin if he could witness an actual execution to help him with the realism of the event and promised him that his attendance there would never be used in conjunction with any publicity connected with the film.
Director Wise only came on board with the picture after Don Mankiewicz had finished the original script. Wise insisted the old screenplay be thrown out and a new one written, He later disagreed with the Writers' Guild decision to give Mankiewicz co-credit.
Virginia Vincent, who portrayed Peg, at the very least diserved nomination for the Academy Award for Best Supporting Actress. When Vincent asked her agent why he wasn't getting her nominated he responding " Not this time ". Her response to her agent was right on the money; " If not now?! When" ?!
The actor who played the Prison Captain of the Death House, Dabbs Greer, would go on to play the elderly version of Paul Edgecomb who was a retired Prison Captain of another prison Death House in the 1999 film The Green Mile. Tom Hanks played the younger version of that character.
The trivia items below may give away important plot points.
Inveterate researcher that he is, Robert Wise was determined to capture every grisly element of an execution for the climax of his movie. He visited San Quentin prison and asked for permission to see the gas chamber and witness an actual execution. After he'd seen it and had his art director photograph it and take measurements for set replication purposes, he was still uncertain about how he would structure the last act. He went back to the prison and made one final request for a detailed account of the entire execution procedure. This is what is painstakingly documented in the movie's climax.
Even though she portrayed Barbara Graham as a tragic victim of circumstance, Susan Hayward later admitted that after doing some extensive research on the real Graham, she was most likely guilty of the murder of Mabel Monohan.
As a reporter for the Los Angeles Daily Mirror, Gene Blake covered every day of Barbara Graham's murder trial, and witnessed her execution and the executions of Santo and Perkins. He called this movie "a dramatic and eloquent piece of propaganda for the abolition of the death penalty." He further stated that the film ignored "...The wealth of evidence which convinced 12 conscientious jurors beyond a reasonable doubt that Mrs. Graham was guilty and should die for a most despicable crime, the brutal and senseless murder of a 62 year-old crippled widow, Mrs. Mabel Monahan."
Joseph L. Mankiewicz, who owned the company producing the film, insisted that there be some anti-capital punishment rhetoric at the end of the film. Wise was sure that if the audience wasn't convinced by then, a few platitudes wouldn't make a difference. Gidding wrote it, and Wise shot it, but in the end Mankiewicz's ending wasn't used.