Nine weeks before filming began, Lionel Logue's grandson, Mark Logue, discovered a large box in his attic that contained his grandfather's personal papers. The box held Lionel Logue's diary, his appointment book, notes from his speech therapy sessions with King George VI, and over 100 personal letters to Logue from the King. It also contained what is believed to be the actual copy of the speech used by George VI in his 1939 radio broadcast announcing the declaration of war with Germany. Mark Logue turned his grandfather's papers, letters, and diary over to director Tom Hooper and screenwriter David Seidler, who used them to flesh out the relationship between Logue and the King. Geoffrey Rush and Colin Firth also read through the material for insight into their characters. The exchange in this movie between Logue and King George VI following his radio speech ("You still stammered on the 'W'." / "Well, I had to throw in a few so they knew it was me.") was taken directly from Logue's diary. Firth insisted that it should be included in the movie.
Lionel refuses to let Bertie smoke during their speech sessions, saying "sucking smoke into your lungs will kill you." King George VI, who smoked 20 to 25 cigarettes a day, died from complications of lung cancer surgery on February 6, 1952, at the age of 56.
Screenwriter David Seidler stammered as a child, and heard King George VI's wartime speech as a child. As an adult, he wrote Queen Elizabeth the Queen Mother (widow of George VI) and asked for permission to use the King's story to create a movie. The Queen Mother asked him not to make it during her lifetime, saying the memories were too painful. Seidler respected her request.
The role of King George VI was written with Paul Bettany in mind. Bettany declined in order to spend more time with his family, and later admitted that he regretted his decision. Colin Firth was cast instead and received an Oscar for his performance.
At 73, David Seidler became the oldest person to win the Academy Award for Best Original Screenplay. He was surpassed by Woody Allen the following year, winning for Meia-Noite em Paris (2011) at 76.