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.
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."
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.
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.
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.