The Other Darrin: There were no less than three different voice actors for Arthur. And it's very noticeable. Two, Richard and Robert Reitherman, were animator/director Wolfgang Reitherman's sons, and sound very similar: young, squeaky, and a lot like Mowgli or Christopher Robin (who were both voiced by their brother, Bruce Reitherman). The third voice, provided by 17-year-old Rickie Sorensen, is obviously the voice of a teenager who's gone through puberty.
What Could Have Been: The Wizard's Duel in the film, judging by concept art, was going to be very similar to the original book, ending with Mim turning into an Aulay (that is a giant equestrian beast with an elephant's trunk) just before Merlin turns into the germ.
Write Who You Know: The animators based Merlin on Disney himself, noting that they were both cantankerous, argumentative, but brilliant and ultimately lovable.