The Masquerader (August 1914)
The best disguise Chaplin has in his makeup box is to take off his mustache. A different mustache, I can recognize him in, but if he's clean shaven, I have no idea who he is. Clearly that's the case of his fellow filmmakers at Keystone, as well.
Chaplin plays himself, famous movie star, who is a bit too into himself and he makes enemies of his fellow actors (Arbunkle has great repartee with Chaplin in his early scene) and his director (Charles Murray) and so is fired. So Chaplin does a Tootsie, dresses as a woman who is so convincing that the director makes a pass tries to rape her. It's a funny gag with some great small moments, including one at the end where Chaplin is fighting with a Keystone staff member and the staff tries to hit him in the face and he keeps ducking... well, you'd have to see it.
In the process of them really improving, this has a coherent story, excellent sight gags, but the ending is too rushed.