Nigel Hawthorne, a stage and TV actor with relative inexperience in cinema, was so keen to reprise his award-winning stage role for the movie version that he took the key part of Dr. Raymond Cocteau in Sylvester Stallone's vehicle 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. In any case Alan Bennett would only agree to his play being turned into a film if Hawthorne was cast as George III.
When Willis first restrains King George in the strapped chair, the music that plays is George Frideric Handel's "Zadok The Priest", commissioned for George II (George III's grandfather) 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.
It is now generally believed that George III's mental state was caused by porphyria, a metabolic imbalance. His blue urine is a key clue. However, recent research into his written correspondence suggesting bouts of mania, and a theory that his blue urine was caused by a type of medicine commonly in use at the time, has challenged that view and asserts that he did indeed suffer from psychiatric illness.
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.)
For this film, Nigel Hawthorne became the first openly gay actor nominated for an Academy Award. (Other actors who later admitted or were later confirmed to have been gay had been previously nominated, but he was the first actor who was already "out" at the time.) He then became frustrated that this was all the American interviewers wanted to discuss, rather than the film or the nomination itself.
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.
The movie "The Madness of King George" is based on a play by Alan Bennett called "The Madness of George III". An urban legend formed that the title was changed to prevent non-British audiences from mistaking it for a sequel to two other movies about "The Madness of George." Nicholas Hytner clarified that in the UK it would be obvious that "George III" was a king, but elsewhere this might not be so clear, hence the name change. (This does not rule out the sequel theory, as the numeral III was not mentioned by Hytner.)
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.)
Fortnum, one of the King's footmen (played by Adrian Scarborough), is shown leaving his service in order to start "a grocer's in Piccadilly". Alan Bennett states in his introduction to the play that he based this character on the historical founder of the London store Fortnum and Mason. Charles Fortnum (1738-1815) was indeed in royal service, but he was a member of the Queen's retinue, not the King's, and the firm had already been founded by 1756 (probably by his father William), five years before he entered her household. Charles combined running the family business with his job as a servant until his death.
Rupert Everett and Mirren both played characters who were blood relatives; Everett as King George VI, father of Queen Elizabeth II in A Royal Night Out (2015) Mirren as Queen Elizabeth II, daughter of King George VI in The Queen (2006)