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According to TV Tropes, Frozen had been in the planning stages of production for a very, very long time. Walt Disney himself actually had some notes on the project, the movie would have originally been produced after Sleeping Beauty. The story back then was certainly much different than what it has become now, most notably that Elsa would have been the primary antagonist (her anger and frustration towards the sorcery-fearing people of Arindelle would have driven her to acts of revenge, though she would have been redeemed in the end). Not to mention of course, that it would not have been CGI.

A cameo of Walt Disney as the king may be a shout-out to the long history of this animated production.