you can see Guy Henry on set as Tarkin - in full costume, no less
I assume that they did Tarkin by filming Guy Henry, in full costume, acting as Tarkin, then replaced his head in post.
I thought the film was great. The third act, as finally released, appears to be a bit of ROTJ-fan-editor’s wishes, in as much as we got ships crashing into the shield, commandos doing stuff and good fighters & capital ship action.