Joan Crawford was approached for this film one month after she left Hush...Hush, Sweet Charlotte 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.
Joan Crawford's role was originally offered to Grayson Hall, who would later join the cast of the enormously popular supernatural daytime soap opera Dark Shadows as Dr. Julia Hoffman, who tries to cure Barnabas Collins of being a vampire.
After Joan Crawford joined the cast her role was meant to be a cameo, although she was given top billing.
During its original theatrical release some theaters installed seat-belts so you couldn't be "shocked out of your seat".
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.
This was Joan Crawford's second (and final) film with John Ireland. Their first appearance together was a decade earlier in Queen Bee.