Was Claire is a difficult role to play, because you are off-screen but must maintain a presence throughout the film?
The book was a help as there was some great stuff about her and Kate Winslet's character, Hester. They're quite sexual together. Hester lies in bed with hot thighs, thinking of her! It was tricky to try and get clues as to who Claire was. I just ended up meeting lots of women who had been at Bletchley. A lot of them were very candid about what went on and the relationships they had.

Did you delve into code-breaking at all?
Fortunately, I'm not a code-breaker. In rehearsals, Dougray, Kate, and I got very confused by it. We got out the machines, but it was they who had to get to grips with it. I wanted to find out about Whitehall and the tactics they used. I discovered the way they behaved when they were vetting people. If they were thinking vaguely of working with someone, they would question the entire community they lived in. I liked that idea that these really young people were at Bletchley but couldn't talk about what they did. It was obviously a very intense environment.

Do you see your character as a femme fatale?
That's the way she's used by Jeremy Northam's character, a way that's realistic to what went on then. A sort of patriotic prostitution almost. It's a weird set-up she's involved in.

How was it working with Mick Jagger, who produced the movie?
He makes me laugh, Mick! He tended to turn up when we were having lunch and entertain us all. He bought an Enigma machine! I've never worked with a producer who was more famous than everyone put together.

"Enigma" is released in UK cinemas on Friday 28th September