The shots of the foundry where Joe works were taken in Avildsen Metals, the family business of director John G. Avildsen.
Peter Boyle won the role of Joe with his ability to improvise in-character during his audition. The producers felt that Boyle was too young (34) for the role and wanted Lawrence Tierney cast instead, but John G. Avildsen insisted that Boyle was the best actor for the part.
Producers fired John G. Avildsen because the final movie ran 150 minutes long and was considered unwatchable by test audiences. They hired William Sachs to come in and try and salvage it. He said in an interview, "The first thing I wanted to do was start in reel five and throw away the first four reels, because it was boring. [Susan Sarandon's character] was with her parents the whole time and Peter Boyle wasn't even in it yet. It now starts fifty minutes into what was the movie. I didn't have money for shooting, but I brought Peter Boyle back, and every time he was off camera I gave him lines. I basically made Joe the main character; he was a minor character before. And I changed the ending. It went on for ten minutes, with everyone discussing what happened." Producers wanted to give him a co-director credit but he refused, so they listed him as production manager. He later regretted that decision because Jack Lemmon was so impressed with the film he hired Avildson to direct Save the Tiger (1973).
Ten weeks before the film was released in the United States, a real-life mass murder with similarities to the movie's climactic scenes occurred in Detroit, Michigan. On May 7, 1970, a railroad worker named Arville Douglas Garland entered a university residence and killed his daughter, her boyfriend and two other students. During pre-trial deliberations, Judge Joseph A. Gillis saw the film and strongly advised both the prosecution and defense teams to do the same. He then carefully screened each member of the jury pool and excluded any who had seen the movie. He also forbade any seated juror from watching the movie or discussing it with anyone who had seen it. Although he brought with him multiple weapons and extra ammunition, Garland received a light sentence. Before and after sentencing, Garland received hundreds of letters from parents across the country who expressed sympathy with him. It was also reported that during the first weeks after his sentencing, he received no letters expressing outrage or condemnation of his actions.