The word sleeper must have been invented for this movie. I can't add much to what the other commentators have said, but this is a movie in the same qualitative category as Judgment at Nuremberg. I'm going to go out on a limb here and take a good guess as to why the movie was not popular. There is a clear if unconsummated homoerotic relationship between the George Segal character and the James Fox character. The latter is really homosexual, but the former is not, though he is a loyal and loving friend. (The medical orderly is also clearly gay.) It is a main thread of the movie, and could not have played very well in 1965, when people either would not have got the point or got it and reviled it.
