Sir 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, Sir 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.
Sir 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 Sir Gary Oldman, twenty-six members of Churchill's family attended the London premiere of this movie, seventeen of whom had earlier visited the set.
Near the end of this movie, E.F.L. Wood aka Viscount Halifax is depicted as saying that Sir Winston Churchill "mobilized the English language and sent it into battle." Although having Halifax utter the phrase can be excused on grounds of "dramatic license", the quote actually originated with American news reporter Edward R. Murrow, who used it in 1954. It was used again by former President John F. Kennedy in 1963, on the occasion of Churchill being given honorary U.S. citizenship.