Of course I’ve seen this before. Fallon doesn’t really have the looks, and certainly doesn’t have the acting credentials. The voice impersonation is good though, like all Jimmy’s acts.
I’ll play
… Given the times and the roles, while other people have played Quasimodo and the Phantom of the Opera, nobody alive in the mid-1920’s was able to do it as effectively as Lon Chaney, makeup maestro and man of a thousand faces.
… To me, it’s difficult to see anybody other than James Cagney play George M. Cohan in Yankee Doodle Dandy - I’m sure that many people at the time, prior to the release, would have had a hard time seeing the usually hard-boiled-gangsta Cagney a fit for the role. So he doesn’t really fit other than in that “You didn’t read the OP” way, but 
I haven’t seen that, but that’s what I’ve heard.
Similarly, Johnny Depp was pretty well spot on in Fear and Loathing. If you watch the documentary Gonzo, IIRC, they have some comparisons and it’s almost hard to tell apart Depp and Thompson.
Now, sure, someone else could have played Hunter in Fear and Loathing, but probably not like Johnny.
Of course, I’d wager that most of the people that watched the movie (such as myself) had no idea who Hunter S Thompson was, much less what he sounded like, what he looked like or what his mannerisms were. It was just a movie about cocaine and acid.
So it’s 1977 and you’ve gotten the green light for a SUPERMAN movie, and you can pick literally any guy on the planet for the title role. No, go ahead; I’ll wait.
Peter O’Toole in “The Stuntman”
And thinking of the actors in that, others may have been able to play Manson in “Helter Skelter” but it’d be tough to beat Steve Railsback.
It’s much more fun to read the list of other actors considered for the part (and cringe): Warren Beatty, James Caan, Burt Reynolds, Kris Kristofferson, Nick Nolte, Robert Redford, Arnold Schwarzenegger, Ryan O’Neal, Jeff Bridges, Jan-Michael Vincent, David Soul, Robert Wagner and Jon Voight.
There are FIVE actors who could have played Indiana Jones - and did.
I think Jordan Prentice could have done it. I used to confuse the two of them before Game of Thrones came on.
Maybe, but William Goldman, who wrote the book, said Andre was exactly what he had in mind for the role.
Sidekickswould not have worked without Chuck Norris. The point of the film was that the kid was training in martial arts and kept fantasizing about Norris helping him out. There was no one else of that stature in the martial arts field (as far as the general public was concerned).
I don’t think anyone else could have played the parts that the Marx Brothers, W. C. Fields, Charlie Chaplin, and Buster Keaton made famous. And no one but Jacques Tati could ever be M. Hulot (at least, not live action).
The role of Jonathan Brewster in Arsenic and Old Lace was written for Boris Karloff (who played it on Broadway). Raymond Massey just didn’t work in the film (there were many jokes about Jonathan killing people because they said he looked like Karloff; the lines fall flat).
Probably the closest I could come is Harold Russell in The Best Years of Our Lives, where he played a serviceman returning home after losing both hands when his ship had been sunk.
Harold Russell actually had lost both hands (due to a training accident, not in combat).
It’s kind of hard to find an actor who not only could act (he received the Academy Award for Best Supporting Actor) and had both hands blown off.
But for roles that (now in beautiful hindsight) we see only one person could play, George C. Scott as Patton sticks out in my mind.
I’m looking to be educated, Dopers.
I was very, very lucky in that when the movies came out, I had never read the Lord of the Rings books, read The Hobbit a long time ago–didn’t like it, remembered almost nothing of it–never saw the Bakshi film, or anything else to give me a visual reference, in short, I had no expectations. I can’t imagine anyone but Ian McKellen as Gandalf. Maybe John Gielgud? Died in 2000. Max Von Sydow? I 'unno. Richard Harris? Too frail looking in the Potter films. Christopher Plummer? Nah.
I’m open to suggestions.
(Pardon the slight hijack)
Seth MacFarlane as Ted. That is all.
Audie Murphy as Audie Murphy in To Hell and Back.
Sean Connery?
I can’t imagine anyone other than John Cleese playing Basil Fawlty.
It’s a very small set of films, and for certain definitions of “only” the answer is zilch. I’ll mention the horrible bit in an Ocean’s Eleven sequel where Julia Roberts creates a disturbance because she looks just like Julia Roberts. But, that dumb joke would work with any famous actress.
The best candidates are star vehicles: the Presley movies, the Beatles films. Star vehicles where the actors are encouraged to do their shtick (and their shtick makes up much of the movie’s content) include:
Robin Williams in Aladdin (animated.)
Jim Carrey’s films, such as The Mask.
Maybe Eddie Murphy in Beverly Hills Cop
The films could have been made with other actors, but would have been so different as to not be the same film.
Maybe the most obvious is Bogart in Casablanca.
Now your talkin’. He’s still in his thirties, though, isn’t he? 
I was referring to Peter Dinklage play Tyriol Lannister, a character who is shaped a great deal by the fact he is a dwarf. Mr. Dinklage is an actor who happens to be a dwarf. It sort of seems required for the part. Certainly, cheaper than the CGI required to make an actor of normal height appear to be half that.
Apparently they toned down the truth becuase the director thought it wasn’t believable for audiences. That is hardcore.