Game of Thrones
Gregor Clegane was one of the most intimadating characters on Game of Thrones, but was that due to his character development or his casting choices? As it turns out, several men have performed for the camera as the Mountain, including Hafþór Júlíus Björnsson, Conan Stevens, and Ian Whyte. His character died at the hands (or fingers, rather) of Oberlyn Martell.
The Joker
Jared Leto's take on the Joker got horrible reviews from critics and audiences alike. However, his taking the role in Suicide Squad meant he would be getting a solo side project. Once the fans criticized his performance though, the deal was off and the enviable side project was given to Joaquin Phoenix. Leto was very unhappy with this decision.