The movie is based on a play by Alan Bennett called "The Madness of George III". The popular story in the UK is that the movie's title is different from that of the play because it was thought the American audience might mistake it for a sequel. While not wholly true, director Nicholas Hytner has confirmed that it was "not wholly untrue" and it is now widely held that this almost certainly did play a part in the titling of the film. In the UK it would be obvious to anyone that "George III" referred to King George III, but it was felt that elsewhere this might not be so clear and that adding "King" to the title might help. While this explains part of the title change, it glosses over the dropping of the "III", adding renewed weight to the original theory.
In reality, the Prince's illegal "marriage" to Maria Fitzherbert did not end until 1794 - about five years after the film's events. (They later reunited for a time after his disastrous marriage to Caroline of Brunswick.)
One bit of business that failed to survive the transition from stage to film: Pitt's drinking. While in the film George III briefly mentions Pitt's drinking habits to his wife, on stage, as Alan Bennett puts it, "Pitt takes a swig from a hip flask, such a regular feature of his behaviour it is not noted in the stage directions." (The historical Pitt was considered a heavy drinker even by eighteenth-century standards, especially as he got older; modern biographers agree that his alcohol intake probably contributed to his early death.)
Nigel Hawthorne - a relatively inexperienced cinema actor most of whose work up till then had been confined to the stage and TV - was so keen to reprise his award-winning stage role for the movie version that he took the part of the villain in Demolition Man (1993) just to prove that he had screen presence. As it transpired this was unnecessary as Hawthorne was the producers' automatic choice for the lead.
There had been some question as to whether Nigel Hawthorne should be cast in the movie, since he was 65 at the time of filming and King George III was only fifty at the time of his first bout of insanity.
When Willis first restrains King George in the strapped chair, the music that plays is Handel's "Zadok The Priest", commissioned for George II and performed during his and every subsequent monarch's coronation. As the music reaches its climax, the King is fully restrained in the "throne" with a leather strap around his forehead resembling a diadem. The music is thus highlighting the restraint scene as a mock Coronation.