Although Walt Disney never knew it, he himself was character designer Bill Peet's model for Merlin. Peet saw them both as argumentative, cantankerous, playful and very intelligent. Peet also gave Merlin Walt's nose.
Arthur was voiced by three different boys - Rickie Sorensen, Richard Reitherman and Robert Reitherman. The changes in voice are very noticeable in the film because of the way Arthur's voice keeps going from broken to unbroken, sometimes in the same scene. One of the easiest noticed is in the last scene in the throne room when Arthur asks in his "changed voice", "Oh, Archimedes, I wish Merlin was here!" Then, the camera cuts farther back and Arthur shouts in his "unchanged voice," "Merlin! Merlin!"
The climactic battle between Merlin and Mad Madam Mim is often cited by animation experts as some of the best character animation to that date. The characters go through numerous physical transformations during battle, yet retain their identifying features; Merlin's guises are blue and include his glasses and facial hair, while Mim's are pink and purple and have her messy hair.
The last film in which Bill Peet served as a writer. He later created a version of The Jungle Book (1967) but Walt Disney threw Peet's story out when the two's relationship fell apart and Peet left the studio.
This was the first Disney film to make extensive use of recycled animation. For example: when Sir Kay is eating the same segment (with different dialogue) is used three times, when Wart falls down stairs the same sequence is used each time, etc.
Two songs written for the film but scrapped before production began were "The Blue Oak Tree" and "The Magic Key". The latter was to be Merlin's lecture to Arthur about the value of an education. It was replaced with the more amusing "Higitus Figitus".
This was the first Disney animated feature made under a single director. Previous features were directed either by three or four directors, or by a team of sequence directors under a supervising director. The man hired for the job was veteran animator Wolfgang Reitherman (one of the fabled Nine Old Men), who would direct all of the Disney features up until the 1980's.