Gamespot has posted an interview with Leonard Boyarsky, lead world designer for Diablo III. In the interview, he explains how they had created the story for the game at the very beginning and have stayed true to this plan. Plus much more about the story behind the game.
GameSpot: With people dissecting the lore, and you as a team cautious of making it too predictable, have you found yourself in the position where you've changed content because players have already guessed how it will play out?
Leonard Boyarsky: No, not really. We figured out what we wanted our story to be, and pretty much stuck to it. If some people guess what we're doing, that just means they'll be pleasantly surprised. Sadly, it's a minority of people who really care deeply about the lore. There are a lot of people out there who do care deeply about the lore, but, in terms of our overall fan base, it's not the top thing on people's list. I think that the kind of people who really care about the lore and delve into that will feel proud if they feel like they figured it out. We haven't seen a lot of people make correct guesses, by the way! [Laughs]