Charlie still hadn't found his partner [for the violin and piano sequence]. At one point he thought Sydney's stand-in, who had a long lugubrious face, could play the pianist. But he was undecided. Then just before shooting, someone told him Buster Keaton was available -- that he was also broke, and needed money. That did it. Charlie hired Keaton.
Buster arrived on the set wearing his old Buster outfit with the small pancake hat. Charlie took him aside and said gently, "We're not playing our old characters now. I'm not playing the Tramp; you're not playing Buster." Keaton, like an obedient pupil, replied, "Yes, Charlie, of course." and removed his hat and went to wardrobe for a costume.
Before our picture began, all the technicians had been excited about working with Charlie Chaplin. He hadn't made a film in years and of course he was a legend...But after a few weeks of shooting, Charlie Chaplin became just another actor. Now their affection switched to Keaton. He was the new boy in town. But if Ben Turpin had shown up two weeks later, I'm sure Keaton would have been dropped like a hot potato. Charlie must have been aware of the technicians' attitude, but chose to ignore it. He just wanted to get on with the business ahead.