[Queen Elizabeth] is just the granny queen! She's our granny queen who shakes people's hands!

That's exactly how I felt. So getting to suddenly know this young [princess] Margaret, who had this extraordinary life - it was sad and tragic and difficult and sort of astonishing and getting to know her young self was amazing because it completely defied all my expectations.

Some of the biographies [of princess Margaret] were really sensationalist, News of the World sorts, but they were great because they also gave first and secondhand accounts of her at home. The butlers come forward and give little moments, some of which you discard and some which ring somehow true and you use.

Marking the differences between them was really important. It just became second nature. When we were choosing pajamas or something, instantly you'd be able to spot: those are [princess] Margaret, those are [princess] Elizabeth. It became this sort of language, really, of the two sisters.

Once you start [filming] you have researchers who are constantly giving you information. Ultimately it isn't about impersonating or trying to be an image of somebody and more trying to capture the spirit and the soul of the person somehow.

It was always said you couldn't have two sisters less alike. In a way [princess] Elizabeth was always internalizing everything and [princess] Margaret was always externalizing everything, so that became the basis. The storyline becomes about these two sisters: they're fighting for their position or trying to establish their identity in the world alongside each other and in relation to this establishment which only those two were a part of.

My dad is a big extrovert - he's a doctor - but he always loved [William] Shakespeare and he took us to tons of theater.