It's this amazing combination to play, really, of somebody who's actually very fragile and hasn't really grown up properly yet - at least in a healthy environment - and has suffered immense loss with her dad - like that line where she says, [in the words of her father [King George VI], "Yes, 'Elizabeth is my pride but Margaret's my joy." She holds onto it!

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.

I remember somebody saying something to me about Frost/Nixon, when Anthony Hopkins does his famous speech, and the difference in the way Anthony did it was to dramatize, essentially, what was a documentary-style version of that speech. I remember someone saying to me, "There is artistic liberty."

[Princess Margaret] was always trying to radicalize things.

I watched tons of archive footage of princess Margaret and listened to the music she loved; that was really immersive and brilliant.

[Princess Margaret] was loud, an extrovert, an exhibitionist, loved fashion, loved color, loved music, loved drama, loved the theater, wanted to be a ballerina or actress, was always the little one putting on the school plays, and [princess] Elizabeth reluctantly did it and got stage fright.

I knew she was a party girl. The book I liked most on her was called [princess] Margaret: A Life of Contrasts and getting to know her, it was how conflicted her position and her internal life - or self - was. She is so fiercely royal and so fiercely "sister of the queen" or "daughter of the king" because that is her identity and it's all she's ever known. And at the same time she is struggling to push the boundaries and to break away from it, to be different or to modernize the monarchy, to turn it on its head.