SPOILERS FOR ROGUE ONE FOLLOW
Rogue One, the recently released Star Wars prequel, has tons of ties to the original films in the form of returning characters. Some, such as Mon Mothma or General Dodonn, were simply replaced by lookalikes. Others — like the brief cameos from Bail Organa or C-3P0 — simply featured the original actors, or in the case of Red and Gold Leaders, used archival footage cut from A New Hope. But the digital appearances in the film by Peter Cushing's Grand Moff Tarken and Carrie Fisher's young Princess Leia through CGI re-creations have been a point of contention for fans in the discussion surrounding the film since its release.
In an interview with The New York Times, several of the lead producers and animators for Rogue One spoke about their reasoning for why they felt the digital re-creation needed to be in the movie along with some details as to how the effect was actually accomplished.
According to Industrial Light & Magic chief creative officer John Knoll, the process of resurrecting Tarkin requires the use of actor Guy Henry as a replacement. Henry performed an imitation of Cushing's distinctive performance for the film in full motion-capture headgear, which then allowed for his head to be digitally replaced with that of Cushing in what Knoll describes as “a super high-tech and labor-intensive version of doing makeup.” If the CGI didn’t turn out well, there were contingencies: the production team would have had Tarkin appear as a hologram.
Ultimately, it was never a question of if Tarkin would appear, but how. As Rogue One co-producer Kiri Hart commented to The New York Times, "If he’s not in the movie, we’re going to have to explain why he’s not in the movie.”
It’s interesting to note that the Star Wars franchise had previously resurrected Tarkin in Revenge of the Sith. There, however, the role was simply played by actor Wayne Pygram, who bears a resemblance to Cushing. But it seems that with the demands of a much larger speaking role that the filmmakers envisioned for the character in Rogue One, the decision was made to attempt to digitally re-create the genuine article in Cushing than rely on a lookalike.
Rogue One also isn’t the first time that a new Star Wars film has managed to bring back actors from the dead — 2015’s The Force Awakens featured a brief dialogue snippet from Sir Alec Guinness’s Obi-Wan Kenobi during Rey’s vision scene taken from the earlier films.
Hal Hickel, an Industrial Light & Magic animator, commented that small details in the Cushing's original appearance in A New Hope had strong impacts on the overall realism of the digital doppelgänger. Things like using the lighting from A New Hope (which differed from that in Rogue One) "improved his likeness as Tarkin, but it worsened the sense of him being real because then he didn’t look like any of the actors in the scene.” But the animators held to a main goal that “realism had to trump likeness.”
Knoll claims that Lucasfilm won’t be making a habit of using the digital effects heavily in future films, describing the process as being "extremely labor-intensive and expensive," and that the decision was made solely for story purposes. But it’s clear that the genie is very much out of the bottle, and assuming that the company can continue to get rights to reuse past actors’ licenses from their estates it seems that there’s little stopping Lucasfilm from continuing to bring back characters digitally in future installments.