This film, which is screening as part of the Keaton season at BFI Southbank, is also being shown on January 7th and 12th. Details here.
Chicago Reader review:
This 1924 comedy finds Buster Keaton anticipating most of the American avant-garde of the 70s: he plays a projectionist who falls asleep during the showing of a detective thriller and projects himself into the action. Keaton's appreciation of the formal paradoxes of the medium is astounding; his observations on the relationship between film and the subconscious are groundbreaking and profound. And it's a laugh riot, too.
Here (and above) is an extract from the film.