Trawling through Hollywood musicals before Gold Diggers of 1933 is a fascinating job. Asides from Lubitsch and the operetta-film, the most salient feature of films like Sunnyside Up (1929) and Follow Thru (1930) is the slenderness of their plots, which are willowy and attenuated in the extreme. Of course one expects musicals to have rather lightweight, simplistic storylines, but these movies extend rudimentary narrative conceits farther than one would think possible, coasting on pure charm.
In today's cinematic world, the art of the musical looks hopelessly difficult: how do you maintain enough story tension to keep the audience hooked, while suspending plot for minutes at a time to indulge in musical numbers which tend to capture the mood of a moment, extending it well past any narrative requirement? In the 30s, they not only did it regularly and effortlessly, they didn't