I agree, but would argue some characters are more definitive than others. For example, I loved Javier Bardem as Anton Chigur (sp.?) in No Country for Old Men, but I could see another actor effectively playing that role. (Christopher Walken in his prime?) Same with Josh Brolin in same film. Dude, Walter, Donny, Brandt…

Or Buscemi's Donny. Or Julianne Moore's Maude. The one I could see as replaceable would be Tara Reid. Lindsay Lohan could probably do that role, for example.

Jeff Bridges as The Dude. Unsure if that's been mentioned, but there will never be another actor more perfect for that role.