Yes, of course the character would have been different but that doesn’t mean it wouldn’t have been a worthwhile performance. We don’t know. I think part of the skill of being a casting director is seeing what an actor might bring to a role.
Wait a minute, Doc: a-are you telling me you built a time machine …out of a DeLorean?
I think it’s kind of tough to come up with a lot of iconic characters played by multiple actors, outside of comic book films, remakes, and “younger versions” of characters. Probably the reason so many remakes and “young so and so” films don’t do well.
Please. Ceaser Romero barely ranks above Lego Joker. Heath Ledger is the standard now. And I would argue that Michele Phifer is the iconic Catwoman.
I don’t think I could ever get used to Cosmo Kramer mutating into Adrian Monk. 
My iconic Catwoman is a six-foot-tall dominatrix wielding a whip. 
BATMAN: What will we do with Robin?
CATWOMAN: (Shrugs) We’ll kill him.
Peter Seller is Inspector Jacques Clouseau.
Alastair Sim is Scrooge. All others are pretenders.
Each Joker matched their respective Batman in style. Cesar Romero was a camp Joker to go along with Adam West’s camp Batman. Jack Nicholson was a quirky Joker to go along with Michael Keaton’s quirky Batman. Heath Ledger was an insane Joker to go with Christian Bale’s insane Batman (although Ledger was much more charismatic than Bale.)
While I thought 60s TV show was fun, it wasn’t really Batman, and Romero wasn’t really the Joker.
With all the talk of the best Joker, I think Gene Hackman had the best
movie portrayal of Lex Luthor.
And of course Terence Stamp’s performance was the best General Zod.
Charlie Chaplin as the Tramp
I’ve got two that are interesting, because they were not meant to be played by the actresses who ultimately played them.
Sarah, on Upstairs, Downstairs, was played by Pauline Collins, whose career was made by this role. She went from “unknown” to “household name” in a season by doing such a magnificent job in this role. It’s impossible to imagine anyone else. She was the “breakout character” on the show-- albeit, I don’t think that term existed yet.
What’s unexpected, is that the role was specifically created for another actress, named Eileen Atkins, who, due to a previous contract she thought had ended, but was suddenly invoked, became unavailable. Atkins is a completely different type from Collins. She at least 10 years older, and about 3 inches taller, with a different look. She was a partner with the actress Jean Marsh, who played part of a pair with the Atkins character. The intent at first was to have Marsh and Atkins performing as a duo throughout the series, but when Atkins became unavailable, Collins was hired for the pilot, but then the character was to be written out. However, she tested so well with initial audiences, that she had to be re-hired and written back in.
The title character of Xena, on Xena, Warrior Princess– actually, a 3-episode arc on Hercules, that then became a back-door pilot when the character was hugely popular with test audiences-- was not intended for Lucy Lawless. It was written for someone else who also suddenly became unavailable (IIRC, she was injured, but I might be misremembering). So the part was offered to three other actresses, one who initially took it, but then cancelled. The crew finally decided to offer it to Lawless, and they were down to the wire, about to start filming, and couldn’t locate her-- it was before cell phones were ubiquitous. They got hold of her with like, a day to spare.
Can you imagine anyone else as Xena?
I came in to post this. Although McKern is the only one I know who has portrayed Rumpole on-screen, others have performed as him on audio. None of them, including the great Michael Hordern, are convincing i n the role. McKern IS Rumpole. John Mortimer wrote most of the stories with him in mind. Heck, McKern performed Rumpole in audiobooks for three different companies.
I also think that Paul Scofield is sir Thomas More in the play and movie A Man for All Seasons. I’ve seen others play the role*, and I’ve seen others play More (In Anne of a Thousand Days, for instance), but Scofield blows them all away.
*Frank Langella played the parton Broadway for a limited run, which I would’ve loved to have seen. I did see Langella in another role that Scofield originated – he played Salieri in Amadeus, and it was great.
And Miss Fritton.
When saying Gandalf’s lines in my head I always used Walter Houston’s voice. It wasn’t until I rewatched The Hobbit cartoon that I realized why. He’s the voice actor who plays Gandalf in the movie!
Well, no. Jeremy Brett was much closer to the original in every way, especially his relationship with Watson. Most actors who play Holmes get that wrong.
True, but Roberto Benigni as his replacement, Jacques Gambrelli was extremely underrated.
That’s not the question, though. Rathbone’s depiction has become iconic, even if other actor’s may be closer to the literary Holmes. It’s very unfortunate that Nigel Bruce’s depiction of Watson has also become fixed to a degree, so much so that when Jude Law depicts him as a capable man of action as in the original stories, many people see it as some kind of ret-con.
In a similar vein, Addams Family Lurch is Ted Cassidy.
And, there will never be a “Curley” like Jerome Lester Horwitz.
But a pretender to the throne would at least have help from makeup. By contrast, imagine the balls it‘d take to say, oh, yeah, no problem; I could hit it out of the park as the Fonz.
I’d like to see a brooding Fonz played by James Dean.