Ok, i have been reading the thread all day, and have come to the conclusion that a lot of people are very precious about their movies and actors. I see very little in most of the performances or characterisation listed to suggest any of these roles could not be tackled just as well by an equally talented actor.
There is nothing much in the many of the roles that are *that* out of the ordinary, that go beyond the bag of tricks that many a great actor of that type would be able to pull off.
In other words...you never know.
So, I have been scratching my head trying to think of a character that was written and played to a degree of uniqueness that any other actor would be extremely hard pressed to strike a balance they could maintain in the role, without overplaying any of the characteristics that made the character great, and all I could come up with was...
Kyle McClachlan as Agent Cooper in Twin Peaks.