According to the “Icons Unearthed” documentary, Lucas came up with the “Vader is Luke’s father” idea before Brackett’s draft was written, which is interesting. I assume that for whatever reason, he didn’t tell her—maybe the idea hadn’t been 100% finalized by that point? Though I haven’t actually seen the documentary (I’m just going off what I’ve heard from others), so although it’s clear that he had come up with the twist before Brackett’s script, I don’t know how finalized the idea was. I suppose it’s also possible that Brackett knew about the twist but ignored it, but it doesn’t seem likely that she’d ignore such a major plot point.
The more I hear about this “Icons Unearthed” show the more I want to see it. I believe that’s the one where Marcia Lucas recalls a cheeky anecdote that George might have gotten the idea for Darth Vader being Luke’s father from a joke made by Indiana Jones and the Temple of Doom writers Willard Huyck Gloria Katz.
The thing that sticks out to me about that story, though, is that she said Lucas was trying to come up with a twist, and what interests me about that is I’ve had a suspicion for a while now that at some point in writing Star Wars - some time after the moment he “split the story in half” and before he condensed back into one film again - that Darth Vader killed Luke’s father was going to be a major revelation (and I think this might’ve been the origin for the title “Revenge of the Jedi”).
If I’m right about that, it could mean the decision to make Darth Vader Luke’s father was not only so he could reintroduce the “cyborg father’s noble sacrifice” plot point from earlier drafts, but also because with the first film he’d ended up blowing what would have been a great dramatic moment if he’d known he’d have the opportunity to use it, and he was motivated to find something to replace it.
Anyhow, what the show apparently says about Brackett jibes with what we know of what Lucas told her about Darth Vader (sorry I know this is like the third time I’ve posted this):
He says more or less explicitly that there’s more to Vader than he’s telling her (saving it for the next film, apparently). Whether it’s “he’s Luke’s father” or “he’s going to turn good” or something else at that point we can’t say.
And yeah, he definitely didn’t do Brackett any favors by withholding whatever his plans at that moment were, because I think the biggest weakness of Brackett’s draft as a story (so aside from not really getting the feel of Star Wars right) is that she’s clearly having trouble getting a handle on Darth Vader as a character, struggling to find a throughline for his various motivations. What she seems to settle on is that Vader is insecure - first he’s obsessed with killing Luke because Luke’s victory over the Death Star humiliated him; then he wants to to turn Luke to his side because he’s tired of being afraid of the Emperor.
I think what Brackett is missing, what she needed to hear from Lucas, is that it’s Luke himself that has awakened a tiny part of Vader’s dormant humanity. This is {his son/ the last living remnant of the old friend he betrayed}, he must meet this boy, the soul he didn’t realize he still had demands it.