The rehearsal is centered around Durante and Toler, and somewhere along the line, someone (possibly the film's editor) noticed that the film's star was missing. Thus the insert shots appear of Keaton, with a neutral background, looking interested in the goings on and saying nothing, so when he does appear near the end we get the impression he has been in the rehearsal room the entire time. The "Singing in the Rain" gag is still cute, and was featured in the compilation film, That's Entertainment (1976).
Of course we had to snip here, and there:
In 1932, it ran 6 minutes, 30 seconds.
In 2021, 5 minutes, 59 seconds.