"Joseph Frank Keaton spent his youth in his parents’ knockabout vaudeville act; by the time he was eight, it basically consisted of his father, Joe, picking him up and throwing him..."

"... against the set wall. Joe would announce, 'It just breaks a father’s heart to be rough,' and he’d hurl Buster—already called this because of his stoicism—across the stage. 'Once, during a matinee performance... he innocently slammed the boy into scenery that had a brick wall directly behind it.' That 'innocently' is doing a lot of work, but all this brutality certainly conveyed a basic tenet of comedy: treating raw physical acts, like a kick in the pants, in a cerebral way is funny. 'I wait five seconds—count up to ten slow—grab the seat of my pants, holler bloody murder, and the audience is rolling in the aisles,' Keaton later recalled. 'It was The Slow Thinker. Audiences love The Slow Thinker.' A quick mind impersonating the Slow Thinker: that was key to his comic invention. The slowness was a sign of a cautious, calculating inner life. Detachment in the face of disorder remained his touchstone.... It was only when Joe started drinking too hard and got sloppy onstage that, in 1917, the fastidious Buster left him and went out on his own. It was the abuse of the art form that seemed to offend him."

From "What Made Buster Keaton’s Comedy So Modern?/Whereas Chaplin’s vision was essentially theatrical, Keaton’s was specific to the screen—he moved like the moving pictures" by Adam Gopnik (The New Yorker).