Cary Grant considered his acting in this film to be horribly over the top and often called it his least favorite of all his movies.
Cary Grant donated his entire salary, $100,000, to the U.S. War Relief Fund.
Ronald Reagan and Jack Benny were offered the role of Mortimer Brewster, but turned it down. Bob Hope was offered the part and was eager to do it but Paramount Pictures refused to loan him out to Warner Bros. for the project.
Alberta Gary, Joan Leslie, Juanita Stark and Vera Lewis are in studio records/casting call lists for this movie, but they did not appear or were not identifiable.
According to various sources, Julius J. Epstein and Philip G. Epstein adapted the screenplay "with help" from Howard Lindsay and Russel Crouse.
Director Frank Capra enlisted in the U. S. Army Signal Corps in 1941 during filming. He received an extension of his order to report for active duty until late January 1942 so he could finish editing the picture.
In the play, Mr. Witherspoon, the caretaker of Happydale Sanitarium, is himself poisoned with a glass of wine by the two aunts, but the censors forced the studio to change this because it would have meant that the two aunts would have gotten away with murder. However, Edward Everett Horton, who played Witherspoon in the movie, did pose for a publicity photograph with a glass of wine in his hand.
Mortimer's repeated phrase at the end of the film declaring the secret of his birth was originally "I'm not a Brewster - I'm a bastard!" However, the censors demanded that it be changed, resulting in the phrase "I'm the son of a sea cook!"
On stage, Boris Karloff played the monstrous Jonathan Brewster, Raymond Massey's film character, who, in eerie-looking screen makeup, resembled Karloff, which was a running gag throughout the picture. Because Karloff was still appearing in the Broadway play during the film's production, he was unable to do the picture.
Some 20 years before filming this movie, actress Jean Adair had helped to nurse a very sick vaudeville performer named Archie Leach back to health; by the time she was asked to reprise her Broadway "Arsenic and Old Lace" role as Aunt Martha for this film, Adair and Leach, now known as Cary Grant, were old friends.
The Broadway comedy opened at the Fulton Theatre on January 10, 1941 and ran for 1,444 performances, closing on June 17, 1944. Repeating their stage roles in the movie were "Brewster siblings" Josephine Hull, Jean Adair and John Alexander, all three getting time off from the New York play. Boris Karloff was denied permission to go by the play's producers, fearing that the absence of their main star would adversely affect the play's attendance.
The film was shot between October 20 and December 16, 1941. During 1943, the film was shown to the Armed Forces overseas. but went unissued domestically until its Manhattan debut at the Strand Theatre on September 1, 1944, followed by the nationwide release on September 23. Warner Bros. had been contractually required to wait for the Broadway play to finish its run, which finally occurred on June 17, 1944. By the time the movie opened, Priscilla Lane and Warner Bros. had ended their association.
When Mortimer is sitting in the graveyard, one of the tombstones has the name Archie Leach on it. Archie Leach is Cary Grant's real name.