The early marketing kind of implied that Luke might go to the dark side , first, in the title ,Revenge Of The Jedi ,before it was changed , then in the first trailers where we see him all dressed in black , like Vader . Plus one of those trailers had a shot of Luke and Vader side by side in the elevator .You could not see any binders on Luke’s hands in that shot . And at that point , it was not confirmed that Vader was his father . As a kid in 1980 , I didn’t believe it and thought Vader was lying .The truth came as a shock to me , but the blow was lessened due to a friend of mine letting me read his Marvel Super Special adaptation of Return Of The Jedi ,which actually came out a few weeks before the movie .