Many stutterers overcome or find ways around their disfluencies, sometimes at a young age and sometimes much older. Many of the world's greatest orators (Winston Churchill, James Earl Jones, etc) had stammers as children. To be the devil's advocate, perhaps Firth had a slight tendency towards stuttering that he overcame as a boy, and by getting into the character of King George, those tendencies resurfaced somewhat. Fluency changes during life. Personally, I thought I'd more or less conquered my stutter, but it's resurfaced over the past few months. Or he's full of bullshit as you suggested. Honestly, I don't really care that much. :)