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 a villain 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.
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 "out" at the time the nomination occurred.) He then became frustrated that this was all the American interviewers wanted to discuss, rather than the film or the nomination itself.
The red dispatch box in which the Prime Minister carries papers for the monarch to sign dates from Victorian times. The first PM to use it was William Gladstone around 1860.