Gary Oldman spent a year studying Sir Winston Churchill and his mannerisms before starting on this movie.
In a sad coincidence, Sir John Hurt was ill with cancer when he was set to portray Neville Chamberlain, Britain's ousted Prime Minister, who was dying of cancer in 1940. However, in an interview, Gary Oldman said that because Hurt was so ill, he never made it to a reading and never got to film a scene. The movie was still dedicated to Hurt, as it would have been his final cinematic project.
Gary Oldman revealed on The Graham Norton Show (2007) that he smoked thirty thousand pounds sterling worth of cigars on-set (about twelve cigars a day) while in character as Churchill, developed nicotine poisoning, and had a colonoscopy during the Christmas filming break.
According to Gary Oldman, twenty-six members of Churchill's family attended the London premiere of this movie, seventeen of whom had earlier visited the set.
The British historical characters were almost without exception played by British actors. However, Australian Ben Mendelsohn was cast, in addition to his several acclaimed prior roles, because he has a close physical resemblance to the real King George VI, more so than Colin Firth and Jared Harris, two actors who had recently played him, and he is capable of a seamless English accent.