Alphonso DeNoble, who played the Spages' fat pervert landlord, wasn't a professional actor at the time. He was working as a bouncer at a gay bar and director Alfred Sole persuaded him to play the role. Before his death in 1978, he got acting jobs in two other low-budget horror films.
Columbia Pictures was set to release the movie (as "Communion"), but pulled out for legal reasons. When Allied Artists picked it up, director Alfred Sole demanded a title change so that the audience wouldn't think it was seeing a religious film. The book adaptation retains the film's original title.
Original prints of this film had the title credits with the animated painting of Alice holding the knife that newer digital copies of the film have, but the original title letters still said "Communion", which, in 1998, was edited to comply with the name it was to be marketed under.
The knife used in the film was created by Alfred Sole's neighbor, who was an engineer, and was designed with a retractable blade. It was the only knife used in the film; the filmmakers never made any duplicates.
The trivia items below may give away important plot points.
In the scene where Dom is attacked by the killer in the stairway at the abandoned building, it was required for the killer to drop the knife after stabbing Dom once and have it land directly on the handrail below, sticking straight up. Director Alfred Sole only had one knife and had to drop it repeatedly, and then have a crew member run back down the stairs to retrieve it and try it all over again. Finally, after over 20 tries, the knife landed directly on the handrail, giving the filmmakers the shot that was needed.
There is a scene halfway through the film were Alice grabs a kitten by the neck, twists it the air, then throws it toward the floor out of the camera shot. This was cut from the British release due to the strict animal abuse laws in the UK.