ET learned even more about the behind-the-scenes magic that went into Rogue One: A Star Wars Story at the place where it all happens -- Industrial Light & Magic, a division of Lucasfilm responsible for the visual effects you see, not only in all the Star Wars films, but in hundreds of other movies as well.
In the final cut of Rogue One, there is a scene on the planet Jedha, where K-2SO (played by Alan Tudyk) slaps Cassian Andor (Diego Luna) in the face to keep up his Imperial facade. As Kaytoo turns to Cassian and says, "and there's a fresh one if you mouth off again," you can see Luna starting to suppress a laugh and covering his stifled smile with his hand.
"It's great," Tudyk said with a laugh. "You see just the corner of his mouth going up."
"The slap was actually Diego Luna's idea," Tudyk told ET. "But the 'and there's a fresh one if you mouth off again' came from me, and we changed that one up a lot because Gareth [Edwards] wasn't totally on board with the slap. He was like, 'Just do it once.' So it was all just fun and games at that point."
Visual effects animator Hal Hickel, who we can refer to as "the mastermind behind K-2SO," emphasized why having Tudyk on set, rather than on a motion-capture stage later on, was pivotal to this moment.
"When you're doing a CG character (if it's a human-sized character), it's so important to have the actor there on set," said Hickel, adding, "Because of all that interaction, and the ideas that come up and the reacting to each other, it's crucial."
Fans can re-watch this moment for themselves because Rogue One: A Star Wars Story is available on digital now and on Blu-ray April 4.