James Stewart had no plans to attend the Oscar ceremony the year he was nominated for this film. Just before the ceremony began, he received a call at home "advising" him to slip into a dinner jacket and attend the ceremony. He did and he received the award for Best Actor. This was in the days before an accounting firm kept the Oscar voting results secret.
The film was shot in eight weeks, and required no retakes. During the scene where James Stewart hiccups when drunk, you can see Cary Grant looking down and grinning. Since the hiccup wasn't scripted, Grant was on the verge of breaking out laughing and had to compose himself quickly. Stewart thought of hiccuping in the drunk scene himself, without telling Grant. When he began hiccuping, Grant turned to Stewart saying, "Excuse me." The scene required only one take.
Katharine Hepburn starred in the Broadway production of the play on which this film was based and owned the film rights to the material; they were purchased for her by billionaire Howard Hughes, then given to her as a gift.
The word "Philadelphia" on the Oscar that James Stewart received in 1941 is misspelled. The Oscar was kept in the window of his father's hardware store located on Philadelphia Street in Indiana, Pennsylvania.
Playwright Philip Barry based the character of Tracy on Helen Hope Montgomery Scott, a Main Line Philadelphia socialite famous for throwing lavish parties at her family's 800-acre estate in Radnor, PA. The studio reportedly intended to shoot the film at Ardrossan (the name of the family's estate), but decided against it after seeing the size and scale of the main house and the expansiveness of the estate. The producers reportedly thought that no one would believe that anyone could actually live like that, particularly in America in the 1940s.
James Stewart wasn't at all comfortable with some of the dialog, especially in the swimming pool scene, which also required him to act in a dressing gown. He said at the time that if he'd played the scene in just a swimming costume it would have been the end of his career.
The original play was written specifically for Katharine Hepburn. Playwright Philip Barry wanted to woo the actress back to the stage after she had received disastrous reviews for the play "The Lake" on Broadway.
When Tracy Lord mentions the Chinese poet who drowned trying to kiss the moon's reflection in a river, she is referring to Li Po (also known as Li Bai) who, according to legend, drowned while reaching from his boat to grab the moon's reflection.
Some think, with director George Cukor on board, Katharine Hepburn's first choice of co-star Clark Gable was never going to be a possibility because Gable allegedly had Cukor fired from Gone with the Wind (1939) because he allegedly detested the director's obvious homosexuality. In reality, Cukor was dismissed from "GWTW" because of repeated clashes with the film's producer David O. Selznick, and Gable was simply busy with other projects at the time this film was to be made.
Before shooting the scene where Connor passionately recites his poetry to Tracy, James Stewart was extremely nervous and certain he would perform badly. Coincidently, Noël Coward was visiting the set on that day and, having been asked to say something to encourage Stewart by George Cukor, Coward off-handedly said something to Stewart like, "Did I mention I think you're a fantastic actor." Stewart shortly thereafter performed the scene without a hitch and went on to win the Oscar for Best Actor.
The original play featured a character named Sandy, who is Tracy's brother and the reason for Mike and Liz to come to the wedding. This character was deleted for the movie in order to beef up the character of Mike. There are several references in the film to a brother of Tracy's, but his name is Junius.
The necklace that Dinah says "this stinks" about and later wears to entertain the reporters, is a copy of the necklace from Marie Antoinette's "The Affair of the Necklace". You can see it in Norma Shearer's Marie Antoinette (1938), as well.
The trivia item below may give away important plot points.
In one scene, Jimmy Stewart carries Katherine Hepburn, singing "Over the Rainbow," from The Wizard of Oz. The director of this film, George Cukor, was briefly employed as the director of The Wizard of Oz, setting the look for many of the characters and continuing to advise on the film even after he was fired.