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At the end of Return of the Jedi, Luke has a vision of three persons: Obi-Wan, Yoda, and some other fellow in Jedi robes. I asked myself: “Who is that third person?”, “Why is he there?”, and “How is the audience supposed to recognize him?”
It’s only with 20/20 hindsight—i.e., having subsequently seen Star Wars Episode II: Attack of the Clones and Star Wars Episode III: Revenge of the Sith—do we deduce that it was Anakin when he turned back from the Dark Side moments before his death as Darth Vader.
Seems to me to be able to identify that third person at the end of Return of the Jedi, the audience would have had to have been clairvoyant.