Via the always entertaining and informative Mark Evanier, this clip of Walt Disney voicing Mickey Mouse, opposite actor Billy Bletcher.
Apparently Walt Disney almost never did his Mickey Mouse voice in public, probably for the sake of both his image and Mickey's, so this is rare footage. And it's amazing how deep and rich Bletcher's voice is despite his 5'2" frame.
But what struck me about the clip is that seeing the actors perform really highlights the slow pace of these old cartoons. In today's cartoons, there would be four jump-cuts and two scene changes in the time it takes Bletcher to say "Meow."