Stage 1: Hunt down EVERYTHING
I really ought to learn not to pick people who had 60+ year film careers for my research jags, if I ever intend on sleeping again.
A physical comedian, is more or less what he said.
"The Rink" (1916)
The first thing I see is "perfectionist". Most of those gags involve pinpoint aim and precision timing. Off by half a beat, and you ruin the take, plus you might hurt someone. The second thing, surprisingly, is "dancer". Given the time frame, I would guess he learned in the English music halls, or American vaudeville. Most of the moments you consciously notice are when the poor Tramp finally hits the floor again, but watch that thing and pay attention to all the time he spends not falling over -- Chaplin is stunningly graceful. This is a man who knows exactly what all of his assorted hands and feet, plus the end of that cane, are doing at any given moment. It is awesome to watch.
Pretty unprepossessing fella, physically, if you don't know who he is. Maybe five foot six? Some of his leading ladies are taller than he is, once they put on their heels. It's difficult to tell in the Tramp outfit, where literally everything but the collar and tie are the wrong size for him, but he pops up in bathing costume from time to time -- he's built like an acrobat.
He also pops up in drag, only once so far. The Tramp escapes a vicious fight once, at the cost of his trousers, and bolts upstairs to lock himself in a room that turns out to be occupied by a traveling dress on a dressmaker's dummy. He visibly runs through "OH SHI-- oh, it's a dress, that's no help. ... I can't find anything else to cover up with, though... and it's a really nice dress..." He manages to wrestle it off the dummy and onto himself, pads the front with the owner's pincushion, and is prancing proudly up and down the second-floor corridor outside when the girl he's been after comes upstairs and finds him.
She blinks, cracks up laughing, and when she can breathe again, tells him to go shave while she finds him some better shoes. I think she might actually be one of the actresses Chaplin married.
Chaplin apparently wrote all of these things himself, insofar as he ever had a shooting script, which he didn't. The Tramp gets himself into a lot of trouble, particularly over girls, but is rarely mean-spirited to people who didn't bite him first. A few people declared him absolutely impossible to work with and after collaborating once would never go near him again, but so far as I can tell, these are all also people like Brando, who were notorious for wresting creative control of their films away from the producer and director -- creative control which Chaplin was equally notorious for hanging onto regardless of competition. People who knew him socially, or were just happier to take direction, uniformly declare that he was charming in person, and stuck to that story even after he kicked off in 1977. I am inclined to like him.
I also think I know exactly why they called Robert Downey Jr for the biopic. If the casting department didn't actually ask for him, I would not be surprised if he auditioned and was hired on the spot. He moves very like Chaplin in a way I can't quite put into words, and has as far back as I've seen him on film. He does it in character to the extent that he borrows from himself for whoever he's playing, but even more prominently as himself. There's a passing physical resemblance. although I think his eyes are the wrong color. (I'm not entirely sure I've ever seen a color photo of Chaplin, but on B&W film his eyes look don't look dark enough against the kohl to be brown like Downey's.) But movement would be much more important, especially for someone who had a long film career based largely on pantomime.