Charlotte Gray seems a very physical role, especially with all that cycling you had to do...

I felt like the Wicked Witch of the West, I was on that bike so much. I remember for one scene I had been cycling all day from about seven in the morning until four in the afternoon. This particular take went on and on and on. After we did the final take my knees went from under me. But the crew just got up and left, I think they thought that I was overacting.

Charlotte is Scottish, and later in the film is supposed to be speaking French though the film is obviously all in English. How do you deal with all those subtle accent changes?

It is a dilemma. The great thing to me was that she remain Scottish and not become an Englishwoman. So there's the sense of her being an outsider even when she's in London. There is a language connection and a cultural connection between the Scots and the French. If you analyse some of the vowel sounds they are similar. But it's not 'the accent show'. When you're playing a character like Charlotte, who becomes more herself by assuming another identity, she finds her own voice.

Did you do some research into the sort of people that Charlotte is based on?

I met a couple of Special Operations Executive people. One especially was an incredible life force, five foot nothing, and she was dropped into France when she was 17. It's quite shocking, I couldn't have imagined finding the wherewithal to function under those conditions when I was that age. But it's not very widely discussed. If you're suited to something like the SOE you are not someone who talks very readily about it.