According to William Finley, the record press in which his character Winslow Leach was disfigured was in a real pressing plant (it was an injection-molding press at an Ideal Toy Company plant). He was worried about whether the machine would be safe, and the crew assured him it was. The press was fitted with foam pads (which resemble the casting molds in the press) and there were chocks put in the center to stop it from closing completely. Unfortunately, the machine was powerful enough to crush the chocks that it gradually kept closing. It was Finley's speed and timing that saved him from being seriously hurt, as he got his head out just in time. His scream in the scene was, in fact, not acting.
When Swan (Paul Williams) is adjusting Winslow Leach's (William Finley) voice, the singer is not Finley but Williams. This makes it a little in-joke when Swan announces that the voice is "perfect".
At several key points in the film - especially during the Swanage audition and "orgy" sequences - the singing and character voice work is provided by Betty Buckley. She originally auditioned for a role in the film and was not cast, but director Brian De Palma called on her because of her skill with ADR and voice work. He would call upon her again to do double-duty, when he finally did cast her as the gym teacher, Miss Collins, in his next film Carrie (1976).
The single-edit, "time bomb in the car trunk" sequence is an homage to Orson Welles' famous opening for Touch of Evil (1958).
The "electronic room" in which Winslow Leach composes his cantata (and where Swan restores his voice) is in fact an actual recording studio, The Record Plant. The walls covered with knobs are in reality a huge, custom-built Moog electronic synthesizer. Dubbed TONTO, this instrument was featured on several albums by the pioneering electronica duo T.O.N.T.O.'s Expanding Head Band, and it still exists. TONTO is now on permanent exhibit at the National Music Center in Calgary, Alberta, Canada and is still available for use to any artist.