Spencer Tracy originally wanted a realistic approach, whereby Jekyll would commit violent deeds in a neighborhood where he was unknown after drinking alcohol or taking drugs. He was disappointed that the producers, having bought the screenplay from the 1931 version, insisted on a more traditional approach. He also said he felt his wig and make up as Hyde made him look "ridiculous".
Spencer Tracy was so eager to begin filming that he turned down James Stewart's role in The Philadelphia Story. Tracy was cast despite concerns that at 41 he was too old to play Jekyll.
Spencer Tracy's appearance as Mr Hyde was disguised in cinema trailers for fear that audiences would laugh at it.
Spencer Tracy's performance as Hyde was judged by the critics in 1941 to be inadequate, principally because he was not frightening enough. In addition, Tracy was considered "too American" and too "rough" to be believable as an upper-class doctor in Victorian London. He later received an amusing telegram from Fredric March, the star of the 1931 version, who said that his earlier performance as Hyde was always compared favorably with Tracy's. After watching the film, Tracy confided to a friend that he believed his acting career was over.
Ingrid Bergman felt badly miscast in the movie, a view shared by film critics in 1941.
Despite having not yet met his famous co-star, Spencer Tracy wanted the characters of Ivy and Beatrix to be played by the same actress, Katharine Hepburn, to reinforce the theme of the good and bad qualities in every individual.
Due to the Hay's Code much of the film had to be watered down from the 1931 version. The character of Ivy Peterson had to be changed from a prostitute to a barmaid.
In the original theatrical version, once Dr. Jekyll is first transformed into Mr. Hyde, he walks up to a mirror in his laboratory. As he stares into it he questions his face saying such things as, "It's my face, yet it isn't" This is followed by him exclaiming, "Can this be evil?" In later TV prints, the earlier lines are missing. They're inexplicably missing to this day. Either the negative was damaged or the cuts were made on purpose.
One night Spencer Tracy turned up at Clark Gable and Carole Lombard's second wedding anniversary party wearing his make up as Mr Hyde.
The character of Ivy Peterson was taken not from Robert Louis Stevenson's original novella, but from the 1931 film version. Ivy never appears in the original story, nor, for that matter, does Jekyll's fiancée.
The concept of the two female loves of Jekyll/Hyde's life, aristocratic Beatrix Emery and barmaid Ivy Petersen, actually originated in the original stage version of "Dr. Jekyll and Mr. Hyde", as adapted by T.R. Sullivan for the great 19th century stage actor Richard Mansfield. The Stevenson novella mentions no female love interest of any sort for either Jekyll or Hyde.
The film was a notorious critical and commercial failure when released. Spencer Tracy later said it was by far the least favorite of the films he had starred in, and that his performance was "awful". The New York Times famously described it as "not so much evil incarnate as ham rampant ... more ludicrous than dreadful."
The studio had originally cast Ingrid Bergman in the Beatrix Emery role and Lana Turner in the Ivy Peterson role. Bergman felt the role of Ivy was more challenging and persuaded the producers to switch roles with Turner.
The studio's original choice for the dual role was Robert Donat.
When author W. Somerset Maugham visited the set during the filming, he supposedly watched a bit of Spencer Tracy's performance and asked sardonically, "Which one is he now, Jekyll or Hyde?"