Screenwriter John L. Balderston is credited with the Egyptian mummy theme for the film. At its earliest stages, the film was simply envisaged as a horror vehicle for Boris Karloff with no connection to Egypt at all. Balderston was a history enthusiast and had covered the opening of Pharaoh Tutankhamen as a foreign correspondent. His experiences in Egypt and love of ancient history influenced him to change the setting and plot of the film to feature an Egyptian mummy.
The flashback scenes in ancient Egypt were designed to resemble a silent film, with no dialog, exaggerated make-up and gestures, and a faster camera speed, to suggest the great antiquity of the events portrayed.
Johann later recalled Karl Freund's nastiness to her: "Karl Freund made life very unpleasant. It was his first picture as a director, and he felt he needed a scapegoat in case he didn't come in on schedule, 23 days, I believe. Well I was cast as the scapegoat - and I saw through it right away! Before shooting started, I asked Freund and his wife over for dinner. He told me for one scene, I would have to appear nude from the waist up. He expected me to say, 'The Hell I will!' Instead I said, 'Well, it's all right with me if you can get it past the censors' - knowing very well that the censors of that time were very strict. So, I had him there."
The character name "Imhotep" was taken from an actual ancient Egyptian, but the real Imhotep was the architect who designed the pyramids and - far from being executed in disgrace - was the only Egyptian, other than the pharoahs, who was made a god after his death.
The mummy is never shown walking onscreen in his burial wrappings, despite the rigorous makeup and wardrobe processes which Boris Karloff endured. Karloff is only shown walking as Ardath Bey and without the iconic wrappings.
Throughout the film's production, there was great tension between Zita Johann and director Karl Freund who disliked each other immensely. According to Johann, on the first day of filming Freund attempted to portray her as a temperamental actress who was very hard to work with to the film's producers. Freund told her that she would have to play the part of Anck-es-en-Amon nude from the waist up, to which Johann replied "I will if you can get it past the censors." Freund ultimately never forced Johann to appear nude; however, he did make life difficult for her on set by refusing her a chair with her name on it, forcing her to stand against a wall for hours to keep a dress from wrinkling, and providing her with no protection from lions in a scene which was ultimately cut from the film.
A lengthy and complicated re-incarnation scene, so important to the plot, never made it into the film. This upset many people, including the film's leading actress, Zita Johann, who was a firm believer in re-incarnation.
Several scenes meant to depict the various reincarnated lives of Anck-es-en-Amon throughout history were filmed but cut from the final version of the film. The scenes depicted Anck-es-en-Amon in Ancient Rome, in the European middle ages, among Viking raiders, and in France during the Ancien Régime.
Zita Johann had proved a difficult actress for both MGM and RKO when she signed with Universal to do "Laughing Boy" from a script by John Huston that she admired. When no suitable actor could be found for the lead, she ironically suggested Humphrey Bogart although the studio rejected him. Since Johann had already been paid, she owed Universal a picture and agreed to fulfill her obligation with "The Mummy."
According to Johann Freund's behavior became dangerous as in those days before the Actors' Guild there was no protection. "Finally we reached a climax. Freund had exhausted me so that the crew was furious; they wanted to kill him. 'What the hell is he doing to her?' they'd ask my secretary. Freund never heard of the 12 hour day. Well, very near the end of the shooting on a Saturday night, we had the scene where Karloff shows me my previous lives in the magic pool. Suddenly I keeled over, and I was out for an hour - dead. They couldn't get a doctor - it was 11 o'clock at night - so the crew prayed me back to consciousness."
Henry Victor appears in the credits of the film as "Saxon Warrior," yet he never actually appears in the movie. The Saxon Warrior was part of a long flashback sequence showing all the heroine's past lives from ancient Egypt to the present. The sequence was cut from the final film.
Boris Karloff was virtually unknown when he appeared as the creature in Frankenstein (1931). He created such a sensation that when this was made, only a year later, Universal only had to advertise "KARLOFF....'The Mummy'."
This is the only Universal monster of the time without a fictional antecedent. Large segments of the movie are scene-by-scene parallels of the movie Dracula (1931). An ankh symbol (the ancient Egyptian glyph for "life") is substituted for the crucifix of the earlier movie. Even Edward Van Sloan's character, Dr. Muller, is quite analogous to his Dr. Van Helsing from the vampire film.
When Ardath Bey steals the scroll from the Egyptian Museum, he was supposed to have left powdered skin in the form of a hand-print, as he did when he escaped from his tomb after awakening when the scroll was read. The scene in which the hand-print was discovered was cut from the film, though.
Arnold Gray is in studio records/casting call lists in the role of "Knight" for this movie, but the part was almost certainly a knight in the deleted sequence depicting the reincarnations of Princess Anck-es-en-Amon prior to Helen Grosvenor. Consequently, Gray does not appear in the film as released.
Jack Pierce's make-up was considered so outstanding that "Hollywood Filmograph" journal honored him with a special award which was presented at a ceremony by Karloff himself. After Pierce's death in 1968 the trophy was believed lost. A decade later when a sink was removed from an old make-up studio at Universal, it was rediscovered.
Among other indignities, Freund put Johann in an arena with lions while he and the crew were protected inside cages and for two days he had her stand against a board so there wouldn't be a crease in her dress.
The script for this movie was originally called "Cagliostro", based on the famous French "prophet"/charlatan who claimed that he had lived for several centuries. It was then rewritten to profit from the love of all things Egyptian since the finding of King Tut's tomb, re-titled first "The King of the Dead," then later "Im-Ho-Tep", and only became "The Mummy" just before general release.
The trivia item below may give away important plot points.
The words Boris Karloff mutters to threaten the rescuers are not nonsense. As he extends his fist, he begins saying "kheb khebet..." Loosely translated, they mean 'destruction.' The full passage they are most likely taken from in The Book of the Dead reads "nebt setau, nebt kheb khebet khesef neseni nehem..." ("Lady of terrors, mistress of destruction, which repulses destroyers; delivering from destruction...").