COURTNEY SOLOMON (Director of the D&D Movie) from the IGN archive, January 2000.
Here's the Mystara- and OD&D-relevant parts of the IGN Scifi interview:
Solomon's own favourite character in his playing days was a druid, a character he kept going for four years. "I was so careful about that character I just retired it," he says. "I didn't want it to die." But don't look for this druid in the movie. "I didn't think it was fair for me to take my personal D&D character and just stick it in the movie."
So he didn't actually take the characters and places from the film out of his own campaign, as you might expect. Instead he based the world of Izmer and Sumdall, where the movie, takes place from one of TSR's older, obscurer campaign worlds, Mystara.
"There was a whole empire of mages in Mystara called Alphatia," Solomon explains. "I just sort of loosely based it off of those things, because I wanted generic stuff, to specifically stay away from stuff like Forgotten Realms and Dragonlance, for various reasons," he says, referring to two much more popular and better known D&D game environments, both of which have had huge numbers of novels and game supplements devoted to them; and in fact, a Dragonlance script is also making the rounds of Hollywood at this point.
And while Mystara was actually a setting for the Basic D&D rule sets, Solomon says most of movie was inspired by the more complex AD&D rule set -- a distinction that's really only going to matter to serious role-players.
"I borrowed more from the AD&D rules, because Basic is just too Basic for everybody I think, where races are just like classes and stuff like that," he says. "All of our characters have classes, races, multi-classes like that. We really made an effort to keep all the rules of the game active in the film, and not break the rules of the game.