The old books that Ann Barrett sees lined up in the cabinet are titled, from left to right; Obsessive Acts And Religious Practices by Sigmund Freud, The Worship of Priapus by Richard Payne Knight, The Psychology of Sex by H. H. Ellis, Sin And Sex, Conation Volition, Sex And Celibacy by T. Long, The Anatomy of Abuses by Phillip Stubs, Phallic Worship and Auto Erotic Phenomena In Adolescence by K. Menzies.
The original novel was set in New England, with an American team of psychic investigators. The storyline was changed, to fit the movie's production in England, with British actors playing the investigators.
Early in the film, in her and her husband's bedroom, Ann Barrett is seen holding the novel "Sentimental Education" by 19th century French author Gustave Flaubert. The novel was no doubt deliberately chosen for her to hold by the filmmakers since much of the novel is about passion, sex, and desire, making it fit in well with the highly sexualized and erotic nature of much of the film. It also foreshadows her later scenes and hints at her repressed sexuality.
Actor Michael Gough spent a very brief time on the production, which is why he is uncredited. Aside from his short appearance in the films finale, Gough's only other contributions to the film were a few recorded lines of dialog.
Once James H. Nicholson left AIP to become an independent producer, this and Dirty Mary Crazy Larry (1974) turned out to be his only post-AIP productions, as he was stricken by a brain tumor and died shortly thereafter.