
Keaton, 1924. A projectionist (and unpaid detective) falls asleep and walks into the picture on the screen. The edits dump him from garden to street to lion's cage. Kathryn McGuire is the girl; Ward Crane the rival. The film-within-the-film is the formal joke. He did the stunts. One of them, the water tower, broke his neck; he finished the picture anyway.
The camera is a door. Keaton walks into the screen and the film cuts the ground out from under him: garden, street, cliff, lions. He stays in continuous motion while the world jumps. Then he wakes up and takes his cues from the movie on how to kiss a girl.
The projectionist walks into the screen and the cuts throw him from garden to street to lions. Then he wakes up and learns to kiss a girl by watching the movie. The method is precision: continuous action against a world that will not hold still.
The projectionist walks into the picture and the cuts throw him around. The film-within-the-film is the construction problem: continuous action, changing backgrounds, a body that has to land in each. The water tower broke his neck. He finished the picture. Then he learns to kiss from the movie.