To anyone who thinks the "Vader is Luke's father" business was part of the "plan" all along:

I just finished reading the first draft of ESB (by Leigh Brackett). It is made abundantly clear in that draft that Vader and Luke's father are two different people, and that Vader definitely killed Luke's father as part of the whole hunting-down-and-destroying-the-Jedi thing.

So yeah, the Vader-as-Luke's-father thing was thought up during the writing of ESB, not before.

And the Leia-as-Luke's-sister thing was thought up during the writing of ROTJ (though much earlier in the writing of that film than the father thing in ESB). That came into being when Lucas changed his mind about making Episodes VII-IX, which were to revolve around Luke searching for his long-lost sister. He now had one movie to wrap up the "There is another" plot point instead of four.

For those who call "bullshit" on that based on Leia hearing Luke's call at the end of ESB - Luke was specifically calling out to her. If he'd called out to Lando instead, Lando would have heard him, too. But Luke didn't know Lando at this point, he can't make a Force connection with a robot, and he can't speak Chewie's language, so Leia was pretty much his only option.

Alec Guiness' hesitation while explaining Luke's father's history, I feel, was just Alec trying to express how much that story pains Obi-Wan to tell. It took on the extra meaning of Obi-Wan lying once Vader became Luke's father, and it was just a happy accident that that hesitation was there.