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Originally posted by: Darth Chaltab
Yeah. I think Lucas planned to have Vader be Luke's father all along, otherwise they would'nt have put so much emphasis on him in A New Hope. Lines like "the Force is strong with this one" make more sense in the context of Vader being Lukes father, since Vader is the second strongest in the Force ever. Also, Luke has met Ben Kenobi before A New Hope. Watch the scene again. Luke doesn't introduce himself.
As hard as it seems to swallow by some here on the forum, GL wasn't sure if he wanted to go through with the father-son relationship between Vader and Luke, which was part of the cliffhanger at the end of Episode V. He decided that he would go ahead with this idea while writing Episode VI. I got this from his own mouth in the documentary "Star Wars to Jedi, the making of a Saga", which I have on VHS.
As far as 'signs' of their relationship already shining through in Episode IV...
Luke's father might have been a Jedi knight and murdered by Darth Vader, just as Obi-Wan told Luke. On basis of the information given in Episode IV and the first part of Episode V (before the "I am your father" thing at the end), Luke's father and Darth Vader could just as well be two intirely different persons all together. Luke being strong with the force (as noted by Darth Vader during the Trench scene) could be due to Luke being the son of a former Jedi knight (who is dead for all we know at the time) and not necessarely because he is Darth Vader's son.
Obi-Wan and Luke have never met before (in person) prior to their meating in A New Hope.
When they meet, Obi-Wan knows who Luke is and calles him by his name. Luke however looks at the old man and asks: "Ben? Ben Kenobi?" Not a question you'd ask if you've met someone before.
During a dining scene at uncle Owens farm, leading up to the discovery that the little R2 unit has gone, Luke says the R2 unit might have been stolen and that it claims to belong to someone named Obi-Wan Kenobi. "Maybe he means old Ben Kenobi? Do you know what he's talking about?" Luke appearently doens't know that Obi-Wan and Ben are one and the same person.
This are examples of how well Luke knows Ben (NOT!). More likely Luke knows off Ben instead of knowing him. Obi-Wan however is more likely to know more about Luke. He might have been interested in Luke's walk of life for sentimental reasons, being the son of his former apprentice, or with the purpose to train him as a Jedi.