The line in question came when K-2SO was caught walking with his friends by a Stormtrooper. The Stormtrooper asked K-2SO where he was taking the Rebel soldiers, assuming that the droid had taken them prisoner. Because K-2SO is not much of an actor, his only response was “These are prisoners.” Where are you taking them, droid? “I am taking them … to imprison them … in prison,” he says. When Cassian tries to clarify the story to the Stormtrooper, K-2SO yells “Quiet!” and smacks his friend in the head. Tudyk’s unscripted line was “And there’s a fresh one if you mouth off again.”
Watching the scene, you can see Diego Luna covering his mouth. Practically, this was because his character was just slapped. From a production standpoint, Luna covered his mouth because he couldn’t help smiling at Tudyk’s improv. Luckily for Luna, the film cuts away from his reaction quickly. One had to look very closely to see the actor “breaking.”
Tudyk also improvised another comedic line later in the film. When Jyn is contemplating her rogue mission, K-2SO steps forward and tells her that he supports her. “I’ll be there for you, Jyn,” the droid says meaningfully. The sentimentality of the scene is undercut by a clever moment of bathos, however, as K-2SO clarifies, “Cassian said I had to.” The droid wasn’t becoming warm and caring, he was just wearily following orders. Tudyk was proud of that moment as well.
“Rogue One” isn’t a terribly well-written movie, and the character work is somewhat weak across the board. Tudyk’s levity, however, made K-2SO the best part of the film. We were lucky to have him.