Wilder directed Marilyn Monroe in The Seven Year Itch and Some Like It Hot. He grew to despise her demands for star treatment and her poor work ethic, and thus included the party-girl Monroe-esque character in this film.
Billy Wilder originally thought of the idea for the film after seeing Brief Encounter and wondering about the plight of a character unseen in that film. Shirley MacLaine was only given forty pages of the script because Wilder didn't want her to know how the story would turn out. She thought it was because the script wasn't finished.
Paul Douglas was cast as Sheldrake but died before filming began. He died from a heart attack while eating breakfast in New York just before he was to fly out to the Coast for filming.
In 1968, playwright Neil Simon adapted the screenplay as the book for the Broadway musical "Promises, Promises". It spawned the hit song "I'll Never Fall in Love Again", composed by Burt Bacharach and Hal David. "Promises, Promises" opened at the Shubert Theater on December 1, 1968 and ran for 1281 performances. The first Broadway revival opened at the Broadway Theater April 25, 2010 starring Kristin Chenoweth.
Although Adolph Deutsch received sole screen credit for the music score, the very popular "Theme from The Apartment" was actually a pre-existing piece of music (originally "Jealous Lover", 1949) by British composer Charles Williams, who was known for his scores for British films and BBC radio dramas.
According to Shirley MacLaine on her official web site, much of the movie was written as filming progressed. The gin rummy game was added because at the time she was learning how to play the game from her friends in the Rat Pack. Likewise, when she started philosophizing about love during a lunch break one day, this was also added to the script.
The office Christmas party scene was actually filmed on December 23, 1959, so as to catch everybody in the proper holiday mood. Billy Wilder filmed almost all of it on the first take, stating to an observer, "I wish it were always this easy. Today, I can just shout 'action' and stand back."
This is the first Best Picture Oscar winner to specifically refer to a previous winner, in this case two of them. First Grand Hotel, which Baxter attempts to watch on television but is too long delayed because of commercials. Bud's boss also refers to Bud and Fran having "a lost weekend" together in Bud's apartment, a reference to Billy Wilder's earlier Oscar winner, The Lost Weekend.
This was the last B&W movie to win Best Picture at The Academy Awards until The Artist. Schindler's List which won in 1994 was not completely B&W as some scenes were in color, like the girl in the red and the candle at the beginning.
It was said that while filming the scene where C.C. Baxter sleeps in Central Park in the rain, Billy Wilder had to spray Jack Lemmon with anti-freeze to keep him from freezing.
To get Fran (Shirley MacLaine) to look genuinely startled when her brother-in-law punches Calvin (Jack Lemmon), director Billy Wilder smacked together two pieces of 2x4 during the shoot.
Billy Wilder claimed that he and I.A.L. Diamond already had Jack Lemmon in mind to play Baxter when they wrote the screenplay. In an interview years later, Lemmon confirmed this.
The studio wanted Groucho Marx for the role of Dr. Dreyfuss, but Billy Wilder said no, stating that he wanted an actor with more dramatic weight for the part.
Billy Wilder wrote the role of "Dr. Dreyfuss" for Lou Jacobi. But the producers of Jacobi's Broadway play wouldn't release him to make the film. So Jack Kruschen played the role and received an Oscar nomination. Wilder made it up to Jacobi by casting him as "Moustache" in Irma la Douce after the previously announced Charles Laughton died.
Twelve different cities are mentioned in the movie: New York, Karachi in Pakistan, Natchez, Kansas City, Seattle, White Plains, Havana in Cuba, Pittsburgh, Cincinnati, Reno, Denver, and Atlantic City.
Billy Wilder claimed that Fred MacMurray was a very stingy man in real life and liked to relate an amusing incident from the filming of the picture. In one scene MacMurray was supposed to tip a shoeshine man and the script called for him to flip him a quarter. When Macmurray couldn't get it right during shooting, Wilder suggested using a bigger fifty cent piece. MacMurray objected because, "I would never give him fifty cents - I cannot play the scene!"
Fred MacMurray had just signed a long-term contract with Disney to do family films like The Shaggy Dog and was initially reluctant to do the morally ambiguous character of Sheldrake.