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And here we see the 6’6” Rory McCann as the Hound practiicing with Conan Stevens, who plays his huge brother Ser Gregor Clegane. Note that Stevens is fighting two-handed here. Interestingly, in the novel this scene takes place with Gregor using a more typical longsword—or perhaps a hand-and-a-half-sword—rather than the greatsword he uses throughout the rest of the series.
Either way, though the Mountain is so huge that he can use a greatsword with one hand, he is actually described as using it with both hands in this particular scene.