Transcript for How 'Rogue One's' Princess Leia, Grand Moff Tarkin Were Created

You know, the new "Star wars" movie may take place a long time ago in a galaxy far, far away. But its special effects are straight out of the future. "Rogue one" fans have been dazzled by the big-screen space battles and alien encounters. But perhaps most thrilling was the seamless resurrection of an actor who's been dead for more than 20 years. And spoiler alert, the special effects jedi masters brought back someone else from the original film as well. Here's ABC's Clayton Sandell on how they did it. Reporter: "Rogue one: A star wars story" currently blowing up the box office. And it's the ground-breaking visual effects that have everyone talking. This is the headquarters of industrial light and magic. It's the special effects company that George Lucas founded in 1975 to help him make the original "Star wars." Of course they're doing cutting-edge work for "Rogue one." We want to take you inside. "Nightline" was granted exclusive access inside ilm's San Francisco mothership. Accelerate to attack speed! Reporter: To see how this sci-fi stronghold brought back, and this is a spoiler alert -- two familiar faces from the past. I recognized your foul stench when I was brought on board. Charming to the last. Reporter: Recreating a 19-year-old Carrie fisher before she passed away last week, and Peter Cushing who died in 1994. Prepare for the jump to hyperspace skin form lord vader. Reporter: This monumental cinematic task fell to John knowl and his team. Why was it so important to have darken and Leia in "Rogue one"? Tarken is central to the death star, what it is, what it came to be. Leia, because we end up on "Rogue one" minutes before the beginning of episode IV. Reporter: In the world of visual effects, aliens and spaceships are relatively easy compared to creating believable humans. We look at human faces all day, every day. People are very attuned to seeing anything that looks off. Making digital humans is one of the hardest things you can do. So we were very, very immediately excited and immediately terrified. Reporter: The first step for ilm's artists was binge-watch every tarken scene over and over. We will crush the rebellion with one swift stroke -- Constantly referring back to how did he look, how did he move, how did he smile? Reporter: Underneath is a real actor, guy Henry, who carefully studied Cushing on the set. Lost in the attack. Reporter: It's Henry's voice and performance the ilm artist used as a guide. The original plans for the station are kept there. The process we'd take to create a shot like this, first shoot the live action. Ts G this is guy Henry. He's dressed in full costume. He has what we call a head-mounted camera rig designed solely for capturing his facial performance. Charming to the last. This is the earliest test? The first time we ever saw guy's motion transferred onto guy's model and then put onto tarken's first early likeness. Reporter: The problem is cush's performance and Henry's performance didn't always match. That required painstaking, sometimes frame by frame adjustments, constantly refining the most subtle details you can imagine, like lips. The original plans for the station are kept there, are they not? So if you look at the corner of his mouth, see how they kind of peel apart? That stickiness is a natural quality that lips have. When they're not -- when it's not present, they feel slightly artificial. Reporter: Ilm also caught a huge break when John knoll tracked down an actual live cast of Peter cush's face made for the 1984 movie "Top secret." This was gold for us. Peter Cushing as he appeared at a certain time in his life. All those pieces together give us the power of illusion. He's there, sir. Reporter: Finally, after nearly 18 months of work, digital resurrection of taken was complete. He's there, sir. The original plans for the station are kept there, are they not? Seeing the movie she didn't realize this was a digital recreation. She commented he looked amazingly well for someone of his age. Reporter: The makers of "Rogue one" had the blessing of Cushing's estate but it's drawing criticism of bringing back well-known actors after they're gone. This work was done with great affection and care. Some of the objections I've heard about this is -- makes references to the Fred astaire with the dirt devil. Nothing mistakes the power of a dirt devil. Or Audrey Hepburn selling chocolate. But that's not what we've done here. I'd like to think that the role that we gave tarken in this film is one that Peter Cushing would have been excited and happy to play. Reporter: Eagle-eyed fans will spot familiar rebel pilots from 1977 reappearing in this "Rogue one" space battle. This is gold leader standing by. This is red leader standing by. That was one of the really fun things about the film is that you start to sense we're getting closer and closer to episode IV, you start to see those moments, wait, I know that guy. Reporter: Princess Leia's brief cameo was made possible by more digital doctoring using a Norwegian actress. Fisher once joked it was hard to watch herself in "The force awakens." I got older and no one told me. Then they put me on a screen really, really big. And they put me in hi-def. Reporter: Before she died, fisher gave "Rogue one" a thumbs-up. Did Carrie fisher see the final scene? It was also done with permission of Carrie fisher as well. That she was involved in the process and she saw the final has always pushed the technological NFL. The first "Star wars" film represented revolution in visual effects techniques. I think it's entirely appropriate we be use the "Star wars" films to move the bar. Reporter: For "Nightline," I'm Clayton Sandell in San Francisco. Lucas film is of course owned by Disney, which is the parent company of ABC.

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