The cast & crew first learned of it when they saw the finished film. When we shot it, Vader's line was "You don't know the truth, Obi-Wan killed your father." Only Irvin Kershner, George Lucas & I knew what would be dubbed in later. Agony keeping that secret for over a year! 😩 https://t.co/DcB2lW8AhC

— Mark Hamill (@MarkHamill) May 24, 2020