When making TESB, only Lucas, Vader (voice actor or body actor, idk) and Hamill knew the twist. Lucas told Hamill literally right before they filmed the scene. He also told Hamill that since only the 3 of them knew, if it got out to the public that Vader was Luke’s dad, he would know that Hamill told them. So, at the premiere, even for the rest of the cast and crew it was a big surprise.
