Walter Matthau, who played Oscar in both the original Broadway play and the movie, actually 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 so 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." and Matthau finally agreed.
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