How was cinema's greatest twist kept secret during The Empire Strikes Back?

The answer

During production of The Empire Strikes Back (1980), the scripts distributed to cast and crew contained a decoy line: 'Obi-Wan killed your father.' Only three people knew the real twist - director Irvin Kershner, George Lucas, and Mark Hamill. Kershner pulled Hamill aside just before filming the pivotal confrontation scene and revealed the actual line: 'No, I am your father.' He told Hamill, 'I know it. George knows it. And when I tell you, you'll know it. If it leaks, we'll know it's you.' Even David Prowse, who physically played Darth Vader, performed the decoy dialogue on set. James Earl Jones recorded the real line later during voice sessions. Hamill carried the secret for over a year before the film's May 1980 premiere.