Mark Hamill

Star Wars: Episode V - The Empire Strikes Back: During production, great secrecy surrounded the fact that Darth Vader was Luke's father. Like the rest of the crew, Prowse—who spoke all of Vader's lines during filming—was given a false page that contained dialogue with the revelatory line being "Obi-Wan killed your father." Hamill did not learn of the plot point until just before the scene was filmed, astounding the actor; Kershner advised him to ignore Prowse's dialogue and "use your own rhythm".