Recreating Tarkin was undoubtedly rather like how the CGI skeleton pirates were handled in the first Pirates of the Caribbean.

First the director would shoot a “practice take” with the actors playing skeletons on set with other actors, to get a reference for the lighting & choreography. Then they’d do a second take on set, using only the actors whose roles didn’t require CGI. The undead pirates would also be filmed separately, so the actors could be digitally transformed into skeletons and overlaid into the scene.

But in this case, as a byproduct of the footage shot for reference, an entire alternate version of the Tarkin scenes exists without the need for any CGI whatsoever. I’m sure Disney is smart enough to make use of that.