[Moderating]
Merged duplicate thread. Also, if you’re reporting a duplicate, it really helps to include the link to the other one.
Benedict Cumberbatch as modern Sherlock Holmes. Johnny Lee Miller wasn’t bad, he just wasn’t legendary. Cumberbatch makes it look easy. Also, Jeremy Brett as classic Victorian Sherlock Holmes. I understand Roberet Downey Jr. is taking a third bite at that particular apple and I just wish he wouldn’t.
Hugh Laurie as Gregory House. May nobody ever try to reinterpret that role.
Rowan Atkinson as Blackadder and as Bean. I guess it helps that he created both roles.
Barbara BelGeddes as Miz Ellie. Not even Donna Reed could replace her.
That’s what I was thinking. We can only imagine Actor A in Role B because that’s who we’ve seen in the role, even creating the role. But had Actor C taken the part, we might have been satisfied. Part of the skill of being a casting director is seeing the possibilities before the role is cast.
Ron Perlman as Hellboy
Jim Parsons as Sheldon Cooper
You just can’t tell. Maybe Selleck as Indy might have been even better. Even more iconic.
As for Sherlock, not only can anybody play him, everybody does. Even if they call him House, or Goren, or Luther, or Patrick Jane, or Backstrom, or Raines, or Monk instead of Sherlock, the character of the genius detective with personal issues is near universal.
And besides, everyone knows Basil Rathbone is the definitive Holmes. 
That’s what I said about Cesar Romero. 
Colombo. Oh, I suppose that someone else could have done it but it wouldn’t have been anywhere as good.
I think it is going too far to say any role couldn’t be played by another actor. I reckon for any given role there are half a dozen capable actors that could’ve slotted in and done a great job. In those universes our perceptions would have shifted and we would not be able see it any other way either.
It is probably more reasonable to say which actor completely owned a character to the extent that a remake would struggle to do better.
In that case I’d suggest Robert Shaw as Quint in Jaws. How many actors get to nail that many scenes in such instantly memorable ways?
I’m not sure there’s any part that can’t be successfully played by other actors.
Years ago, people thought that only Robert Preston could play Harold Hill, yet Craig Bierko was great in the revival. Yul Brynner was the definitive king of Siam, yet Lou Diamond Phillips was fine in the role, as was Ken Watanabe.
Superman was always George Reeves, until it was Christopher Reeve. Batman was only Adam West until it was Michael Keaton, until it was Christian Bale.
Noted for future reference, thank you!
James Earl Jones as the voice of Darth Vader
Like Indy and Selleck, I’ll disagree on this one.
I could see a same period Walter Matthau doing a great Colombo, and I could see a current day Jeff Goldblum. You just have to rein him in a bit.
Thinking about it, I’d rather have had Matthau. Unless the glass eye is necessary, to put the suspect off guard, keep them distracted.
Definitely. He does such a good job with the role that I was really hoping the last prequel or Rogue One would have Darth Vader as one of the featured characters. It’s a shame that never happened.
I’m pretty sure Mark Ruffalo could’ve nailed it.
I can see Matthau doing an excellent job, but Jeff Goldblum comes across as too goofy to succeed in the role.
I don’t feel like looking for it but video of Selleck’s Indiana exists, from from a screen test IIRC.
It’s a fine line. At the time of Raines he could have probably done it, now today he just play Jeff Goldblum in any role. 
It could have gone either way. High Road to China is sort of a Indy role, and it isn’t that memorable. Magnum’s Legend of the Lost Ark, I mean, ART was better.
Was it the actor, or the material? Tales of the Gold Monkey was a Indy-inspired TV show, and I thought Stephen Collins was wrong for the part.
Billy Bob Thornton as both Bad Santa and as Karl in Slingblade.
Clark Gable as Rhett Butler.
Marlon Brando as Stanley Kowalski. Many others have done it, but he really nailed it.
Bob Newhart in both of his TV series.
I’m a little surprised no one’s mentioned Michael J. Fox as Marty McFly (Back To The Future), since that’s one that there’s actually empirical evidence for. (They tried making the movie without him, and it didn’t work.)
I’m inclined to agree.
Still, there are some roles that a particular actor did so well, and so idiosyncratically, that it’s hard to imagine anyone else in that role.