The abandoned house was originally just one that director Ray Dennis Steckler had found while location scouting in the Topanga Canyon area of Los Angeles. After being struck by its eerie atmosphere--and frightened when its owner appeared and surprised him-- he decided to write the resulting scene around it.
When the film was originally released, director Ray Dennis Steckler took it out as a roadshow engagement, playing it at midnight screenings and drive-ins. During the murder scenes, Steckler--and others wearing masks designed to look like him--would run out into the audience with prop knives in order to startle them.
The film was meant to only focus on the three mental patients. However, during shooting director Ray Dennis Steckler realized that the film was not going to run long enough, and decided to shoot scenes with him as Click.