I started to really enjoy the fact that [princess] Margaret was an exhibitionist. Even on a day-to-day basis, Margaret's costumes were always so much more dramatic and bold than Elizabeth's were.

Everybody has an image of [princess Margaret], to a certain extent. But I felt it would have been harder if we were playing them as they are now. In a way, I don't know how much of a living memory we as a collective have of them in the '50s, when Margaret was 21 and this sort of Elizabeth Taylor. You don't think of your grandparents as being teenagers. You just can't - your brain just can't go there!

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.

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."

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!

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.

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.