Mel Ferrer also appears at the Eiffel Tower party as Dr. Jekyll and Mr. Hyde.
Tony Curtis agreed to a guest role in the film as a favor to George Axelrod, because William Holden was forced to enter an alcoholic clinic in the middle of filming.
Tony Curtis was a last-minute addition to the cast, apparently added to the film in much the same way in which he suddenly appears in the screenplay.
Hubert de Givenchy was the first to receive a screen credit for a scent. He got screen credit for Audrey Hepburn's perfume.
According to Audrey Hepburn's son Sean H. Ferrer's memoir, this was her least favorite of the films she made during her years as a star, despite the fact that she called it "a joy to make". Ferrer wrote that it taught her a lesson: "Just because the film was easy to make doesn't mean it's going to be very good."
Another Audrey Hepburn movie to be mentioned is Breakfast at Tiffany's, mentioned at the beginning of the masquerade scene.
During one scene, Audrey Hepburn and William Holden compare Frankenstein to My Fair Lady, the latter of course being the film Audrey would soon star in. Charade, Hepburn's next produced film, also contains a reference to "My Fair Lady".
Filming ended only two days before production began on Audrey Hepburn's next film, Charade. The small park with its Punch and Judy puppet stage, seen during the opening credits, appears in both films.
French actor Raymond Bussières is post-synchronized in English by another actor but he post-synchronized himself in the French version.
The Dracula sequence was the last one completed for the film. It had to be shortened because William Holden smashed his new Ferrari into a wall and wound up in a splint.