Sir Nigel Hawthorne, a stage and television actor, had little theatrical movie experience. He was so determined to reprise his award-winning stage role on-screen that he took the part of Dr. Raymond Cocteau in Demolition Man (1993) to prove that he had screen presence. Hawthorne was the producers' automatic choice for the lead. Alan Bennett only agreed to his play being turned into a movie if Hawthorne was cast as George III.
Many historians believe that George III's mental state was caused by porphyria, a metabolic imbalance that can cause blue urine. However, recent research into his written correspondence suggests bouts of mania, and a common type of medicine at the time could have caused blue urine, leading some to conclude that he had a psychiatric illness.
When Willis (Sir Ian Holm) first restrains King George III (Sir Nigel Hawthorne) in the restraint chair, the music that plays is George Frideric Handel's "Zadok The Priest", commissioned for King George II, and performed during his and every subsequent coronation. As the music reaches its climax, the King is fully restrained in the "throne", with a leather strap around his forehead resembling a crown. The music establishes the restraint scene as a mock coronation.
In real-life, the Prince's illegal marriage to Maria Fitzherbert ended in 1794, about five years after this movie's events. They later reunited for a while after his disastrous marriage to Caroline of Brunswick.
This movie made Sir Nigel Hawthorne the first openly gay actor nominated for an Academy Award. Actors who later admitted or were later confirmed to be gay had been nominated, but he was the first actor who was already "out" at the time. He was frustrated that it was all the American interviewers wanted to discuss, rather than this movie or the nomination.
In this movie, King George III (Sir Nigel Hawthorne) briefly mentions Pitt's (Julian Wadham's) drinking habits to Queen Charlotte (Dame Helen Mirren). In the source play, as Alan Bennett puts it, "Pitt takes a swig from a hip flask, such a regular feature of his behavior, 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.
This movie 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 U.K. it would be obvious that "George III" was a King, but it might not be so clear elsewhere, hence the name change. That does not rule out the sequel theory, as the numeral III was not mentioned by Hytner.
The movie is about events of 1788. In the scene where King George III is looking at the globe, America is shown as already possessing the Louisiana Purchase, which didn't happen until 1803, and the Oregon Territory, which wasn't claimed until 1846.
Fortnum (Adrian Scarborough), one of the King's footmen, leaves 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 Dame Helen Mirren 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).