Anna: so how are your TA duties [for Film and Television Music] going?
Sarah: Well, I showed The Adventures of Robin Hood (1938) on Friday. That man was just too happy.
Anna: Did he bound?
Sarah: Um...yes. He did.
Not only did Robin bound, but he also threw his head back and chortled...by the sixth time, the undergrads were tittering, and I don't blame them. The conventions for acting on screen were still being sorted out in the 1930s, and while it is obvious to us that stylized movements are unrealistic and melodramatic, such practices were perfectly legitimate for the stage (and probably still are). The stage actor must make his movements visible to the last member of the back row. So we have conspirators nodding their heads and opening their eyes as wide as possible before winking at each other. The expendable soldiers cringed and flailed before flopping over dead.
Singing in the Rain demonstrates the disasters and difficulties that attended the move from silent film to talkies. Because a lot of silent film actors ended up having horrible voices, directors would take actors from the stage. In addition to having pleasant voices, these actors could also project enough for the infant sound technology to catch their lines. But these actors also took their acting conventions with them.
The music picks out the diagetic elements of the scene: party music in the court, fanfare trumpet music for the archery contests. It also follows what will become conventions (I suppose) for later nondiagetic music: romantic violins for love scenes, fast-paced for action, etc. Music generally supported scenes involving Robin and the good guys. Interesting correlation of morality and music.