|"Hey, what are you looking at?"|
At first, they act within the rules of the game: they've got to imitate the other person, ignore that the imitation isn't perfect, and not break the plane of the mirror. But soon, the rules get broken.
Charlie Chaplin in The Floorwalker, 1912.
The Marx Brothers in Duck Soup, 1933.
Abbott and Costello in The Naughty Nineties, 1945.
Lucille Ball and Harpo Marx in I Love Lucy, 1955.