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!"
Although Walt Disney never knew it, he himself was character designer Bill Peet's model for Merlin. Peet saw them both as argumentative, cantankerous, but playful and very intelligent. Peet also gave Merlin Walt's nose. This was the second instance in which Walt unknowingly served as model for a wizard, the first being the wizard Yensid from the Sorcerer's Apprentice in Fantasia (1940). This explains why the character was given the name Yensid. This read backwards is Disney.
This was the last animated film that Disney produced, since he died during the production of Mogli: O Menino Lobo (1967).
Every time Arthur trips, falls, or has some accident where he loses his footing, the sound recording of his voice is identical to every other incident.
The last film in which Bill Peet served as a writer. He later created a version of Mogli: O Menino Lobo (1967) but Walt Disney threw Peet's story out when their relationship fell apart and Peet left the studio.