Conan Stevens As The Mountain In "Game Of Thrones", Replaced By Ian Whyte And Then By Hafþór Júlíus Björnsson
In Game of Thrones, The Mountain is a character who has been played by a lot of actors. Conan Stevens portrayed him in season one due to his physical characteristics. Ian Whyte then took up the role, becoming him for season two, before getting replaced by Hafþór Júlíus Björnsson. He seems to fit the description almost point to point.
Also, how would anyone be able to tell when some background character in GoT was replaced? There were a ton of them, and they all looked the same.