The film was originally shot in the summer of 1977. However, the original version was considered unreleasable. Director Denny Harris turned to screenwriters Jim Wheat and Ken Wheat to help improve the story, but they had the radical idea to reshoot the bulk of the film. The actors and actresses who played the college students were brought back for reshoots in March 1978. Yvonne De Carlo, Barbara Steele, Cameron Mitchell, and Avery Schreiber were all brought in during reshoots for name-recognition value, replacing the actors who originally played their roles. In the end, only 12 minutes of footage from the original version was retained in the final finished film.
When Rebecca Balding was called back for reshoots 8 months after filming had concluded, she discovered they had discontinued the nail polish she'd worn in the original footage and had difficulty matching the color.
Director Denny Harris was a successful TV commercial director with his own self-named studio who dreamed of making his own horror movie. Utilizing his own staff, facilities (which doubled as the police station) and money, he churned out this independent film.
Murray Langston originally played the part of Mason, which was radically different from the character Brad Rearden embodied. Ken Wheat said Langston played "an offensively exaggerated gay character, very broad, supposedly villainous because he was gay." This characterization was the first thing changed when the Wheat brothers were brought in to overhaul the film.
Cameron Mitchell and Avery Schreiber were hired to work on the film for two days. Because of their short shooting schedule, most of their scenes were covered with two cameras. Without the knowledge of his agent, Mitchell returned for one afternoon to film a few pick-up shots and was paid $500 cash.
Diane McBain originally starred as Lt. Sandy McGiver. The filmmakers considered retaining her footage when the film was completely reworked, but the actor playing opposite her as Sgt. Manny Ruggin had a running gag where he was continuously eating donuts that the filmmakers decided didn't work.
Rebecca Balding originally turned down the role because she had landed a job on Lou Grant (1977). After the third episode was shot, Balding was "let go" from the series and then accepted the part in the film.
The Engels house is actually a Highland Park, Los Angeles home called The Smith Estate, and it was previously known as the Merrye house in the cult classic Spider Baby or, the Maddest Story Ever Told (1967). In both instances the house was chosen because it stands upon a hill, which made it easier to shoot wide angles and obscure the view of the neighboring homes in order to achieve a sense that the house is very isolated.
All of the exterior shots of the Engels house were filmed in a single day. The filmmakers mapped out where scenes would appear in the film and they moved the camera around the house throughout the day, taking full advantage of the sunlight.
During the scene in the basement laundry room, the color of the shirt worn by Juli Andelman alternates between bright and pale yellow. When they returned for reshoots many months later, the filmmakers found the color impossible to match.
The hidden staircase from the basement to the attic was a single set. To achieve the illusion that it snaked up multiple stories, the lighting, cobwebs and furnace pipes were switched, with narrower pipes on the upper floors.
Over the course of filming, three different locations were used for the laundry room scene: a real basement, a small basement set and a smaller set for the dark corner next to the furnace and hidden staircase.
In the original version of the movie, the killer sneaked up and murdered Jack as he lied in bed wearing headphones. Part of this scene was retained in the final cut, but the rest was changed when the film was overhauled.
One scene was shot without permits at Occidental College, and drama students were recruited to play extras. A staff member tried to halt filming but the Wheat brothers quickly got the consent of the school's Dean, who watched off-camera.
The Engels house doesn't really overlook the ocean. To achieve the shot where Scotty pulls back the curtains and looks out the attic window, a curtain was simply hung in front of the camera next to the beach-front staircase seen elsewhere in the film.
The trivia items below may give away important plot points.
Juli Andelman did not like playing background as a corpse in the closet after her character was killed, so she intentionally moved around and ruined takes. Eventually the filmmakers decided to have another character cover her body with a sheet so the actress would not have to be present during the filming of those scenes.
A scene was scripted in which the killer wraps a rope around Doris's corpse and drags her up the hidden staircase, which explains how she got into the attic. The scene was deemed too technical and time consuming to properly achieve, so it was never filmed.