Seriously. If you haven't seen Rogue One, stop now.
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Ok, then. The NYT has a great interview that jumps into how Tarkin was modeled for Rogue One, what went right and what went wrong.
Some snippets below, but you should hit the link and read the entire thing.
Source:
http://www.nytimes.com/2016/12/27/movies/how-rogue-one-brought-back-grand-moff-tarkin.html?_r=1
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Ok, then. The NYT has a great interview that jumps into how Tarkin was modeled for Rogue One, what went right and what went wrong.
Some snippets below, but you should hit the link and read the entire thing.
But as they revisited the 1977 George Lucas movie that started the Star Wars franchise, and gave fresh screen time to some lesser-known heroes and villains, the staffs of Lucasfilm and Industrial Light & Magic faced artistic and technological hurdles: most prominently, using a combination of live action and digital effects to bring back the character Grand Moff Tarkin. This nefarious ally of Darth Vader and commander of the Death Star was played by Peter Cushing, the horror-film actor, who died in 1994.
In doing so, they also waded into a postmodern debate about the ethics of prolonging the life span of a character and his likeness beyond that of the actor who originated the role.
The effects experts and storytellers behind Rogue One (which was directed by Gareth Edwards and written by Chris Weitz and Tony Gilroy) say they have given careful thought to these issues and were guided by their reverence for this interstellar epic.

In striving for a balance between a digital figure who seemed real and one who looked precisely like Cushing, the Rogue One creators said seemingly minor tweaks could make significant differences and these details were tinkered with constantly.
For example, the original Star Wars film (also known as A New Hope) was lit differently than Rogue One, raising questions of how to adjust the lighting on the character.
Hal Hickel, an Industrial Light & Magic animation supervisor, said that lighting him the way he was in A New Hope improved his likeness as Tarkin, but it worsened the sense of him being real because then he didnt look like any of the actors in the scene.
Side-by-side comparisons of Cushings daily footage from Star Wars and Mr. Henrys motion-capture performance also called attention to subtle tics in the original actors delivery.

In her first shot, when Leia is seen from behind (with her trademark hair buns), she is played by a flesh-and-blood actor, Ingvild Deila. Then, in the reverse angle, when Leia is seen from the front, her face, hair and costume are a digital re-creation of Ms. Fisher, based on footage from A New Hope. (The characters extended hand is Ms. Deilas.)
To deliver on that moment of hopefulness, that is really underscored by the fact that you do get to see her face, Ms. Hart said. Thats the best possible use of effects, to enhance the meaning and the emotion of the experience for the viewer.
The Rogue One filmmakers reviewed the dailies from Star Wars and turned up other Easter eggs to add to their movie. They included unused footage of the X-Wing fighter pilots known as Red Leader (Drewe Henley) and Gold Leader (Angus MacInnes), who help wage the rebels attack on the Death Star.
(Some of that original film, Mr. Knoll said, was somewhat underexposed, requiring some digital repair work. The actors were then transferred by rotoscope from that footage and inserted into computer-generated cockpits for Rogue One.)
Mr. Knoll said he did not come across a vast gold mine of footage that Mr. Lucas did not use. George was pretty economical in the way he shot the films originally, Mr. Knoll said. When he was happy with a performance, hed go, All right I got it.
Source:
http://www.nytimes.com/2016/12/27/movies/how-rogue-one-brought-back-grand-moff-tarkin.html?_r=1