In an interview, Elijah Woods is quoted as saying:
"I read the first script [for The Hobbit] and it's incredible. It definitely expounds upon the book; there are already characters that are cast that are not in the book, so that indicates that it goes slightly outside the boundaries in the structure of the original novel."
For the moment, let's put the whole "go[ing] outside the boundaries" bit - Cate Blanchett's character has no business being there, just to start - aside; after all, Peter Jackson did well by The Lord of the Rings though he definitely strayed from the source. Instead, let's focus on the question of just who used "expound" when they (probably) meant "expand".
There's no way to tell if Wood said "expand" and the interviewer put "expound", or if it was Wood's original word, or even if some editor changed it. These days I expect most interviews are recorded, but maybe the reporter was taking written notes and misread her writing, or misheard Wood, or just wrote the wrong word down in the first place - it's terribly easy to write something you didn't mean. If it was on tape, there's the opportunity to mishear. At that point, though, you'd wonder how secure she was in knowing the meaning of "expound" - unless she just thought Wood didn't know... But it's also quite possible that Wood's pronunciation of the two words is very similar, or even that he said one meaning to say the other, or said one thinking it meant the other.
Errors like this are very hard to analyze. It's an easy misspelling, an easy mispronunciation, an easy slip of the tongue. And it might just be a mislearned word with no production error involved at all. But we can't even be sure whose error it was, can we?