Hafþór Júlíus Björnsson plays an HBO character who first appears in book form in what year?
Hafþór Júlíus Björnsson:  He plays Ser Gregor "The Mountain" Clegane in the HBO series "Game of Thrones". Gregor Clegane:  In the books, the character is initially introduced in 1996's "A Game of Thrones".
1996