I don't get why they're doing that. They're doing research on how Cushing walks, not Tarkin. He could walk with a limp and it wouldn't matter, regardless of the "distinguishing gait", I'm sure there is someone who could pull it off without fisting the CG angle.
They could use the guy they used for the end of Revenge of The Sith, they didn't seem to mind how he walked back then.