Meg Tilly was never allowed to watch any sort of television as a child, and so had never seen the original Psycho (1960) and was unaware of its significance. She didn't understand why the press was giving all the attention to Anthony Perkins for his comeback role in this movie, and one day on the set Perkins overheard her say, "Why is Tony getting all the attention?" Perkins was upset, didn't talk to her during filming, and recommended that she be replaced, even though half of her scenes had already been shot.
When Anthony Perkins was ambivalent about the film (and leaning towards passing on returning to play Norman Bates), rumors flew that Christopher Walken was going to be cast in the lead role and the film would be a TV movie event. However, Perkins eventually agreed to star in the film. In the DVD commentary for the movie, screenwriter Tom Holland was asked if Walken had almost become the new Norman Bates. He said he could not confirm or deny the reports and immediately began talking about an unrelated subject.
The way that Norman says "cutlery" in the scene in the kitchen was conceived during a table read of the script. Anthony Perkins accidentally stuttered when he said the line and director Richard Franklin loved it and told him to say it like that in the film.
Meg Tilly's character's name, Mary Samuels, is a reference to the original Psycho (1960). In that film Marion Crane signs her name as Marie Samuels in the Bates Motel. The book upon which the film was based had Marion named "Mary".
Writer Robert Bloch published the novel "Psycho II" in 1982. The plot of the book is very different than the feature film. It has Norman Bates escaping from the mental institution and traveling to Hollywood, California to stop the production of a film based on his life. Universal Studios was reportedly upset by Bloch's take on the horror film industry and this lead to the development of this 1983 film sequel.
The scene in Norman's childhood bedroom where Mary comforts him on the bed was added by screenwriter Tom Holland at the request of Anthony Perkins, who felt that his character needed a moving moment with the character of Mary Loomis.
Director Richard Franklin was a Hitchcock student - they first met when Franklin ran a Hitchcock discussion/retrospective during his first semester at USC - and even visited him on the set of Topaz (1969).
Sheriff Hunt was originally called Sheriff Chambers and Dr. Raymond was originally called Dr. William Richmond in the script. They were both characters from the original film but their names were changed at the last minute.
The trivia items below may give away important plot points.
The final scene of Norman hitting Mrs. Spool with the shovel was not given to cast and crew until the last day of shooting. All the shooting scripts ended with a message saying: "The final scene will be distributed to cast and crew on the last day of shooting". The only people that knew about the ending were director Richard Franklin and writer Tom Holland.
The iconic last shot of the movie with Norman standing in front of the house was used as a Christmas card for various crew members. When Universal presented concept art for the one sheet film poster, director Franklin wasn't pleased with it. It was editor Andrew London who came up with the idea of using the Christmas card photo as the film poster and also came up with the famous tagline.
Death count: 6 (It could be argued that it was only 5 as an extended scene shows that Mary survived the gun shot wound. Alternatively it could be argued as 7 as the infamous shower scene from the original movie was shown as a flashback at the start of the movie).