The onetime Sergeant Pink sees the Doctor as a soldier himself: a commanding officer who expects absolute obedience from those who travel with him – even to the point of putting their lives on the line. This is clearly too close to the truth for comfort.
For the first time, we get a glimpse of the Doctor’s guilt, doubt and perhaps even self-loathing that haunts his latest incarnation. His rage at Danny addressing him as ‘Sir’ is naked; ugly, beyond doubt – something rather new for modern fans of Doctor Who. But it’s a measure of Capaldi’s brilliance how quickly and fully his acerbic, snarky performance had been accepted.”