- All-Star Cast: Quite an impressive list of talent for a 60s British horror film. David Niven, Deborah Kerr, Donald Pleasance, Flora Robson and while it was on the shelf - David Hemmings and Sharon Tate also became quite well-known.
- Billing Displacement: David Hemmings is given higher billing than his supporting role would indicate. This is because - while this movie sat on the shelf - he had his Star-Making Role in Blow Up. It's thought that his newfound fame, and that of Sharon Tate (see below) was what convinced studio execs to finally release the film.
- Contractual Obligation Project: Kim Novak was in the role of Catherine because she had signed a three picture deal with Martin Ransohoff.
- Fake Brit: Sharon Tate was Texan born but put on an English accent quite convincingly to play Odile.
- Follow the Leader: Was one of many films in the 60s dealing with human sacrifice, such as The Witches (1966), The Devil Rides Out and City of the Dead.
- Hostility on the Set: According to David Hemmings, tensions ran high between Kim Novak and producer Martin Ransohoff after the former's injury (see below). This may have been a factor in her replacement.
- The Other Marty: Catherine was originally played by Kim Novak - who had shot most of her scenes when she fell off a horse. Despite only having two weeks of shooting left, her back injury was so severe, Deborah Kerr had to replace her.
- Production Posse: One of several films Sharon Tate starred in that was produced by Martin Ransohoff.
- Romance on the Set: David Hemmings and Kim Novak had an affair during filming.
- Self-Adaptation: Robin Estridge wrote the original novel the film is based on, and adapted the screenplay. However the novel is credited to his pseudonym Philip Loraine.
- Star-Making Role: The film was intended as one for Sharon Tate, but it was released after Don't Make Waves (hence the 'introducing' credit for her).
- Typecasting: Deborah Kerr's part here bears a lot of similarities to her other horror role in The Innocents. Such things include her being a newcomer in a Big Fancy House with suspicious servants, dealing with a creepy brother and sister, trying to protect two children in her care, suffering Sanity Slippage and being powerless to stop someone from dying in the end. Some camera angles seem to homage The Innocents as well.
- Urban Legend of Zelda: Wild rumors circulated for years that Sharon Tate had been introduced to Wicca by former black magician Alex Sanders - who acted as an on set consultant.
- What Could Have Been: The film changed directors twice. Sidney J Furie was first signed on but he left right before filming started. His replacement was Michael Anderson, who in turn fell ill and was replaced with J Lee Thompson. Right before filming, the title was also temporarily changed to 13.
Trivia / Eye of the Devil