Joan Crawford was approached for this film one month after she left Com a Maldade na Alma (1964) due to an "ailment" that prevented her from working (which is believed to have actually been sick of working with her arch enemy Bette Davis). Therefore, William Castle requested that Crawford's doctors sign a statement attesting that she was completely well before giving her the role.
Joan Crawford was paid $50,000 for four days work. Her total screen time amounts to just 9 minutes. That amount of money would be equivalent to just under $500,000 in 2023.
This was Joan Crawford's last appearance in an American film. After this, Crawford made only two more films, both of which were filmed entirely in the UK.
During its original theatrical release, some theaters installed seat belts so you couldn't be "shocked out of your seat."
Director William Castle used a gimmick, as usual in his features, to promote the film. He placed a plastic phone near each theater entrance where audiences, mostly youngsters, were asked to dial a number, which was supposed to reach a message saying, "I saw what you did and I know who you are." But the phone lines were rapidly jammed and complaints from phone companies all over the country led to Castle eventually canceling the whole operation.