You've spent a long time turning interviews down. Was that because you thought you would just be asked about your mother?
Yes. I think I was almost too cagey about my mother. I hadn't dealt with it very well. I didn't really know how to, so I went completely undercover. I got an agent on my own and got into the National Youth Theatre on my own. I did it all on my own and that is important when you are trying to find your own way. Now I like to get the mother stuff out of the way in an interview. She is my ma, not Diana Rigg and that's it. But I am so proud of her and it took a long time for me to acknowledge that in public. I felt it would take away from what I did.
It clearly hasn't though. You're about to be seen in ITV's stunning update of Othello. You play Lulu, best friend of the doomed Desdemona. There are some very intimate moments with Christopher Eccleston, who plays Iago. How do you prepare for scenes like that?
You sit down and talk about it first and you laugh a lot. Then you have 500 people around and a camera up your right nostril. You don't see nudity, but in a way I suppose it is more intimate than that. They never have full sex because I don't think Jago - as Iago is called in this - would let himself go like that. Jago and Lulu are quite similar animals. If he hadn't been so fixated with Othello they might have fallen in love. Lulu seems your average spoilt kid, but she has a lot of dignity to her and strength. She is street-wise and slightly sordid in a way that Desdemona isn't. She has learned about men the hard way. She had this great line: "I used to sh** people out of sheer politeness. They only had to say 'Hi, Lulu' and I would have my knickers down".
Oo-er, but Lulu is still quite upper-class. Your dad Archie Stirling had an estate in Perthshire, Scotland. Are you posh too?Well, I speak with a posh accent...
Not too many plums in your mouth though.
No, (laughing) not too many plums. It has always been perceived that I grew up in a large estate like something out of a Jane Austen novel. I didn't. My father is often described as a landowner, but I think many farmers will tell you owning land is not about living in luxury and being profligate - now it's more about survival. I know compared to many I have been very lucky, but it wasn't the silver-spoon upbringing that's often made out.
You went to boarding school in England when your parents' marriage was breaking up (her father had an affair with the much younger Joely Richardson). Was that your choice?
Yes it was. I was given a choice of schools and from the age of 11 to 18 I went to Wycombe Abbey girls school in Buckinghamshire where I had an incredible education. It is one of the top schools in the country and they give you the confidence to believe that you can do and be, anything. That is the privilege of private education, but it ain't all jolly hockey sticks. The first week we were dragged up to the attic and put through a sort of initiation by the older pupils. The 16-year-olds brought out a tampon and asked what it was. I put my hand up and said it was a candle and they collapsed giggling. They put us through tests to see what we knew. We were petrified.
But they didn't do anything cruel to you?
Well, it was all about power, so it was cruel in a sense. But at the same time, no sooner are you grown up than you can't wait to turn the tables and do the same thing to the younger ones. We were also made to "fag" - where the younger girls do chores for the older ones. It was proper Cruella De Vil-style stuff, but at the same time you are all bright-eyed, bushy-tailed and eager to please. I was like: "Oh, really? What can I do for you? Shall I wash your pants?" Boys' bullying is well- publicised, but girls' bullying is much more psychological - more malicious. It is an almighty thing to go away from home to school and be disillusioned at the age of 11 and a half. I am not sure that I would send my own children to a boarding school. I was OK because I was equipped to deal with it, but not all children are.
With the benefit of hindsight, would you have been happier at home?
No. I would have had a s**t time at home.
Because of your parents' divorce?
No - nothing to do with that. You go through a weird patch when you start going through puberty and it's fine because you have spent your fury and your frustration at the structure which is boarding school and its staff, who represent authority. By venting your feelings on them, instead of your parents, your parents remain important, separate and sacred. When I got my period I phoned mum. She said don't worry everything's going to be fine. The next day I got a bunch of flowers from her and a note saying "Welcome to womanhood", which was lovely.
After school you studied Russian and art history at Edinburgh University and did two films while you were still there?
Yes, that was amazing. The first was Still Crazy, with Bill Nighy, Timothy Spall, Juliet Aubrey and Jimmy Nail. The director asked me if I would quit university. I said no and so they flew me down to Pinewood for filming and back for lectures. I played Juliet's daughter and fell in love with a young guy in a rock band. My performance was awful. It took me 10 takes to walk through a door, but I learned a lot. I was absolutely terrified in the company of such accomplished actors.
Hadn't you met any of them through your mother?
Don't be daft. I had never been to a celebrity party in my life. Nobody even knew who my mother was until halfway through filming when a newspaper ran a piece about me being "undercover" on the film. I went on to do Complicity with Jonny Lee Miller. Then after graduating, I did Another Life about Edith Thompson, who was hanged for a murder she didn't commit, and Maybe Baby, the Ben Elton comedy.
Joely Richardson was in that too. Wasn't that rather awkward?
When I first heard I had the job and that Joely was in it I thought, "Oh God!" But I never thought about backing out because it was work and I needed the money. We met working on the film and it was hard, but OK. It was nine years after the event so a long time had passed and we sat down and talked about it - I faced my nemesis and we got on. Also, it's not like we are the only people this has happened to. Lots of people live with adultery and marriage breakdowns. It happens all the time.
You once said you were unconvinced by the idea of a man and woman being together forever. Do you still feel that?
No. I have flitted around a bit - I always have. I flitted and then I met the boy. I think I probably am having it - real love - for the first time. I never believed there was someone out there for me, but now I do. We always knew each other to say hello to, but suddenly one day I let go and the flood gates opened and it has been exhilarating, amazing. We have had more than six months of that absolute joy of getting to know someone. Letting them know all your neuroses and then they still love you and even laugh at you when you get the green-eyed monster.
Who is this perfect man?
John Green. He is a DJ in London - he works in clubs and does everything from rock to hip-hop and he's my boy. He is 23, a year younger than me - I know we are young, but I really feel I have found my soul mate. I know this business is tough on relationships but I feel he will cope.