One of the main things with Tarkin was that the CGI Tarkin was really designed, ahem, quite differently from the on-screen character as he appeared. In the film there was overexposure, deep shadows, the highlights were blown out due to harsh light, and he generally looked like an old film photograph (which it was).
The CGI Tarkin had a very equalized, HDR appearance with constantly shifting eye & facial muscles and a very even reception to light, which did not seem to be emanating from anywhere. There's also a weight-free fluidity of movement, as in a modern video game.
What's funny if you watch the real Cushing in the film is that he himself had far less facial muscle movement than the CGI version and moved in a faster manner. The CGI version, I hate to admit, looked like an alien of some sort from the prequels.