Walter Matthau, who played Oscar in both the original Broadway play and the movie, asked the play's author, Neil Simon, if he could play Felix instead. This was because Matthau thought Oscar's personality was too similar to his own and the role would be too easy; whereas playing the persnickety Felix would be a real acting challenge. Simon replied, "Walter, go and be an actor in somebody else's play. Please be Oscar in mine." Matthau finally agreed to it.
The two great friends, Jack Lemmon and Walter Matthau, are paired in a movie for the second of ten times.
Monica Evans and Carole Shelley reprise their roles in Disney's The Aristocats (1970) as a pair of English geese on a walking tour of France.
When getting ready for their dinner with the ladies, Oscar jokingly asks, "You think Mozart goes good with meatloaf?" Walter Matthau was a passionate Wolfgang Amadeus Mozart enthusiast, and had extensive knowledge of his work.
According to former Paramount production chief Robert Evans in his memoir "The Kid Stays in the Picture", producer Howard Koch originally wanted to use the Broadway cast, Walter Matthau (Oscar) and Art Carney (Felix) in the movie. Evans wanted Jack Lemmon for Felix. Evans also wanted Billy Wilder, who directed Lemmon and Matthau in The Fortune Cookie (1966), as writer-director. The cost for the Lemmon-Matthau-Wilder package was $3 million plus 50% of the profits. Paramount owner Charlie Bluhdorn balked at the demands and personally took over negotiations. Wilder eventually dropped out. Lemmon was signed for $1 million against 10% of the gross and Matthau got a straight salary of $300,000.