Umm, Coward's play is called Quadrille, I'm pretty sure; I've got a copy around here somewhere. Kind of sentimental, as I recall. In the movie Quartet, Maggie Smith moves into a retirement home for performers, where she finds the residents planning a production of Rigoletto -- so it must be THAT quartet...plus (I'm guessing) Maggie and three other characters in some non-singing ensemble.