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.
Columbia Pictures was set to release the movie (as "Communion") but pulled out for legal reasons. When Allied Artists picked it up, director Alford Sole demanded a title change so that the audience wouldn't think they were seeing a religious film. The book adaption retains the film's original title.
Linda Miller was injured while filming the scene in the hospital, and production had to be halted for a month. Fortunately, she didn't have to go far for medical help since they were filming in a hospital.
The trivia item 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.