The Odd Couple

This entry refers to the play and movie and not the subsequent TV series.
Neil Simon wrote the play after his brother Danny had made attempts at writing it but never made it past page 15.
Danny Simon had recently divorced and started to live with the also recently divorced theatrical agent Roy Gerber. Their life together formed the basis to what eventually became The Odd Couple, Neil Simon more polarising the two leads.
Neil Simon had wanted to cast Walter Matthau as Oscar after seeing his portrayal of Nathan Detroit in a Broadway production of Guys & Dolls. It would be the first of many appearances for Matthau in a Neil Simon work.
Matthau actually thought the role of Oscar was too close to him and wanted to play Felix as this would be more of an acting challenge. Neil Simon wrote in his autobiography "Rewrites" that he told Matthau to "act in someone else's play. Do Oscar in mine".
With Art Carney cast as Felix, The Odd Couple was a Broadway hit in 1965 and continues to be produced in amateur and professional productions around the world. It wasn't without its initial hiccoughs though and the third act had to be rewritten several times, even after press night, where the reviews were favourable, but many commented on a need for a stronger third act, which was soon written.
The success of the play (Simon's third) gave rise to the 1968 film, with Walter Matthau reprising his Oscar role and Jack Lemmon in the Felix part. This was Matthau and Lemmon's second starring film together, following the 1966 Fortune Cookie, it is said that had Matthau not won an Oscar for The Fortune Cookie at the previous year's Academy Awards he would have won one for The Odd Couple.
The screenplay was adapted by Simon and apart from adding some extra locations (the entire play being set in Oscar's apartment) the script remains largely the same as the play.
The simple basis of the story has led to many spin-offs and rewrites, the TV series with Jack Klugman (who first played the part on Broadway immediately after Matthau) amd Tony Randall, an African-American TV series, a cartoon (The Oddball Couple) and a female version of the play. Matthau and Lemmon also reprised their roles for The Odd Couple 2 in 1998.
So here is the original theatrical trailer, I hope if you haven't yet seen this fabulous movie you will now seek it out and even better catch a local production, I'm sure it will be appearing at a theatre near you some day soon.