From Wikipedia: Garry Moore recalled, "I asked (Keaton) how he did all those falls, and he said, 'I'll show you.' He opened his jacket and he was all bruised. So that's how he did it—it hurt—but you had to care enough not to care." This would have been in about 1955, when Keaton (born 1899) was an old man and well past his heyday of really dangerous stunts (he once broke his neck during an early stunt).
And he usually had an amazing commitment to film in a lot of other ways. The first time he was shot in a film he took a camera apart to figure out how it worked, because he really cared about every detail (though in the middle of his career this really hurt him, as execs wanted to just trot him up in front of the camera as a high paid celebrity - they didn't want him wasting his valuable time fussing over details, or risk their investment letting him do stunts).