In a fascinating 1960 interview with Studs TerkelKeaton explains in brief detail how subtitles are done during the silent period.
“Your lips move. You speak. And in the cutting room, you just run the movie through your fingers down to where you just opened your mouth, and on the second syllable, you’ll cut, add to your subtitles, [which] explain what you are talking about. And then when you turn around, you pick it up just as your mouth is about to close. So that’s the way it is. “
However, they like to convey plot with action, and this is where Keaton competes with Chaplin. Film length has expanded from five reels to seven, so movies now run between 75 minutes and an hour and a half. . Who is the winner? “I think Chaplin won that,” Keaton admitted. “He finished one of his paintings [in] something like 21 titles and I’ve got 23.”
https://www.slashfilm.com/984387/buster-keaton-challenged-charlie-chaplin-to-a-silent-movie-showdown/ Buster Keaton challenged Charlie Chaplin to a silent battle of wits