Secondary interpretive sources!
Chaplin (15th Anniversary Edition)
It's more than a little unnerving to watch an actor whose tells I've come to know pretty well play another actor whose tells I've come to know pretty well, and get it right. Particularly when the party of the first part is still having a lot of his infamous drug problems, and the party of the second part isn't around anymore to complain about anything they got wrong.
His kids were, though. Geraldine Chaplin, eldest daughter, played her own grandmother Hannah, and one of the sons was involved enough to interview for an early VHS extra. Apparently they started out going, "Well, I guess he looks a little like our father, sorta," and then by the time he came over the hill in full costume to film a scene with Geraldine, she caught her breath and went, "Oh dear God, it's Dad."
Downey's impersonation would be almost frightening if I didn't think he felt it was a genuine honor to have the part. Observe (embedding annoyingly disabled). If this scene occurs in any of the Chaplin/Normand shorts, I haven't found it; it seems to be a pastiche with blocking specifically meant for a modern multi-camera set. From the time Downey shuffles out of the wardrobe in the battered size 14 shoes, he looks remarkably like Chaplin in behind-the-scenes footage; and from the moment he rolls his shoulders back while walking to the set, he becomes Chaplin in character as the tramp. Right down to flipping the hat back onto his head before taking a photo with the bride. Breathtakingly eerie is the description that comes immediately to mind.
Of particular note is the way he jumps around on the scenery like he weighs nothing. It's a trademark of Chaplin's to bounce around on his toes as if gravity is unconcerned with his existence; I've seen Downey do it too, usually when doing very physical comedy, but also in less overtly slapstick things, like poinging around on the equipment in Stark's home lab. Men don't typically like to look weightless. Possessing mass, commanding space, and having a solid stride are all ways of creating a presence, especially if you aren't built like a tank. If you don't have sufficient force of personality, removing your footfalls removes you from wider awareness. RDJ has charisma to beat the band, and he knows it, and I suspect it is one of the things that contributes to this fascinatingly un-self-conscious portrayal of another extremely charismatic man.
I probably sound like I'm simpering. Eh. I like uncommon people and uncommon things, and I particularly like seeing them cross paths. Add to it that both of the people involved are beautiful to watch at their work, and you get over a week of babbling.