Queen Elizabeth II had blue eyes, like her first portrayer Claire Foy. After Olivia Colman was cast as Elizabeth she was tested with blue contact lenses over her brown eyes, but it was decided they negatively affected her performance. Changing her eye color in postproduction was also tested, but according to the producers "it didn't feel like her. CGI-ing her eyes seemed to diminish what she was doing". Eventually it was decided to accept the continuity error.
The series is one of the most expensive television series ever made. Each episode is budgeted at £5 million and it had already been commissioned for two seasons, with the intention of four more, before the first episode had even been completed, or any episode broadcast.
The iconic black door of 10 Downing Street had to be made larger in scale so that John Lithgow, who is 6' 4'' (1,93m) in height, would not look significantly larger than the real-life Sir Winston Churchill, who, at this stage was around 5' 6'' (1,67m) tall.
For his role as Sir Winston Churchill, John Lithgow's dialect coach was William Conacher, who also had to coach the other mostly British cast because of subtle changes in English accents over the decades. Lithgow actually stuffed cotton in his nostrils in order to capture the faint nasal timbre of Churchill's intonations, while a dialect evolutionist was on set to monitor the accuracy of accents over the time span of the series. It was noted, for example, that Queen Elizabeth II's pronunciation of vowels during the 1950s differs enormously from the way her grandson Prince Harry speaks today.
Coincidentally Helena Bonham Carter's uncle, Mark Bonham-Carter, briefly dated Princess Margaret, the very character Helena portrays in Seasons 3 and 4 of the series.