Which actor/actress was so perfectly cast for their role that it would be impossible to see anyone else playing the role?

Maybe for you, it's Michael J Fox as Marty McFly. Maybe it's Harrison Ford as Indiana Jones. Maybe it's Robert Downey Jr. as Iron Man or Sylvester Stallone as Rocky.

So, for you, who do you think was so perfectly cast that it would be hard for you to accept another in that character role?

Just an FYI, it doesn't have to be a favorite movie or anything like that.
Which actor/actress was so perfectly cast for their role that it would be impossible to see anyone else playing the role?
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