they really went to town when creating the beast reading this article about the film...
"Essentially, Stevens had to give two different performances — one neck-down and one neck-up. He acted out the movements of his character’s body in takes shot on the film’s sets, using stilts to enhance his height, and wearing a lycra body suit with tracking markers which would facilitate a CG rendering of the Beast’s torso and limbs. Then, at a later date, Stevens’ face was covered in ultraviolet makeup and actor replayed the scene seated in front of a bank of cameras. This footage which would then be used to create the Beast’s face, using the so-called “Direct Drive” process, a new technology developed by Digital Domain. There was the physical puppeteering and the facial stuff,” says Stevens. “Essentially, you go from these incredibly lavish, amazing, tangible practical sets on these stages at Shepperton [Studios] — and everything’s looking gorgeous and there’s me looking like a crash-test hippo on stilts — and you’ve got to do all that again, but you’re now essentially in Tron. I’m sat wearing a black t-shirt in a sort-of UV booth with 27 cameras. The whole thing felt very very magical in a way. It just felt like pure magic how they fused these two experiences together."