auximenies said:

Does anyone know how ADF went about writing this? -- zombie84? Was he given guidelines? Did he have the old screenplays and free reign to use ideas that didn't make it into Star Wars?

Foster today insists that he had a free hand and basically made it up himself, but that cannot possibly be true. For one, it contains the usage of deleted plotlines from Lucas' own drafts, such as the primitive aliens defeating the Imperials--a major section of Lucas' first screenplay--and the kiber crystal. So at the very least he was working in part from Lucas' early material. Additionally, there are quotes from earlier in time when Foster says that he and Lucas together hashed out the plot. Lucas also had veto power--Foster's outline had begun with a space battle, but Lucas told him to cut it out so that if they ever adapted it into a film they could do so on a low budget, using props and costumes from the first film that Lucasfilm had held on to.

But in addition to that, Lucas must have sculpted much of the plot himself in personal story conferences--for one, we have partial transcripts of such things. In late 1975, Lucas met with Foster--Lucas had three films in mind, which he had in his contract with Fox, as most people know, but he doubted that they would ever get made given the unsuccessful progression of pre-production at that point in time. So he hired Alan Dean Foster to do them as novels. He outlines some basic ideas at this point--in book two there is to be a love triangle with Han, Leia and Luke, but Han leaves at the end and Luke stands poised to get her. He calls it Gone With the Wind in Space. He also says that Vader's past will be dealt with, and in part three the Skywalker family history will come more into play. But this was a very early concept--script changes and personal changes would necessitate story changes in these sequel novels in the subsequent months. Harrison Ford was not signed to a contract, so he does not appear in book two (SOTME), and then Lucas cut out the lightsaber battle between Luke and Vader from Star Wars, so this would become book two's climax; probably, he also included the backstory of Vader in Star Wars, that being he was the murderer of Luke's father, which he might have originally saved for the confrontation in the sequel. The Kiber crystal was also still part of Star Wars at that time, but then was cut out for the final script. So by the time Foster actually got around to outlining book two in early 1977 it had changed quite a bit, so the concepts from the previous drafts filled out the plot--the kiber crystal and battle of primitives--plus other ideas that Lucas had in mind for a sequel like a swamp planet (Dagobah in ESB) and such, Luke and Leia's romance was now sans Han Solo, and Luke and Darth would have their first confrontation at the end of the story, but without any personal twist.

So, though the in-betweens and details were Fosters, I would attribute more of the elements in the story to Lucas himself, rather directly or indirectly. Before Foster had finished writing book two Star Wars was released and became a hit, so Lucas scrapped book three altogether and decided that the film version of Chapter II would be a totally different and much grander story.