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RobertAltman wrote:...I was under the impression that 180-degree system/rule came about as a result of the sound film, along with the rest of the characteristics of the classic Hollywood style?...

I suspect that the rule was followed long before it officially became a "rule" simply through trial-and-error.

Beginning with his first films as a creative force in 1920, Buster Keaton demonstrated a very keen awareness of camera placement and, even more importantly, screen direction. There is good reason for this: his comedy depended on the audience being able to follow the continuity of the action. If he played loose with the 180-degree rule or paid little attention to screen direction, the audience would momentarily lose their orientation, the timing of the gag wouldn't be as precise and the laughs wouldn't be as loud. Keaton seemed to inherently know how to seamlessly cut from one image to the next. This is especially remarkable when you consider that he routinely combined shooting sites from all over Los Angeles and treated them as one "location" for his action. In his short DAYDREAMS he even inter-cuts footage shot in San Francisco with footage shot in L.A. to create a chase sequence. If Keaton is seen exiting frame left in one shot, you can bet that he will enter the next shot frame right no matter that the two locations are miles apart and shot on different days. You can still find examples where Keaton breaks the 180-degree rule (not to good effect), but these moments are fairly rare.

Keaton is one of the most compositionally fastidious directors you can find, and he takes the entire frame into account when staging a gag, knowing full well that the size of his figure in the frame and the presence / absence of other visual distractions in the frame are vital components: too much visual clutter, or being too close / too far away from the gag, and the joke is weakened. I think it's also important that a lot of Keaton's gags depend on us understanding often very complex spatial relationships. The sublime joke when the cannon goes off while pointed at Keaton in The General works so beautifully because it satisfies our understanding of a whole matrix of spatial relationships: his proximity to the cannon; the angle of the cannon; the presumed trajectory of the cannonball; the speed of the locomotive; the distance between its cars; the speed of the other locomotive; the distance between the two locomotives; the lay of the tracks. And all of these things are established in an utterly unforced and natural way through the clarity of Keaton's mise-en-scene, and not just as a laborious series of set-ups for the payoff, but as an organic part of the storytelling or as factors in previous gags.