Actor Sam Elliott (Pete Danner) and actress Katharine Ross (Margaret Walsh) met and fell in love when they co-starred in this movie. They married in 1984 and are still together.
Two years after its release, Sam Elliott gave an interview to Jerry Buck of the Associated Press in which he said about this movie: "I wouldn't rush out to see it. It's about fifteen years behind its time."
John Coyne was asked to write a novelization in six weeks to promote the movie, and Hollywood spent a lot of money to make it "briefly" a best seller.
Considered for Karl were Harry Andrews, Peter Cushing, Bernard Archard, Herbert Lom, Donald Pleasence, Sir Christopher Lee, Michael Gough, Anton Diffring, Peter Arne, Patrick Troughton, Peter Vaughn, and Donald Houston.
The film's primary writer Jimmy Sangster originally wanted to use the film to get away from typical British horror like his past scripts for Hammer. His original script was set in a run-down hospital in Detroit as opposed to an isolated mansion in the English countryside. The studio instead insisted it be set in the U.K., as it would be cheaper for them to film locally, though they did permit American protagonists. Sangster wasn't happy with the film being more in line with Hammer-type horror films.