Sienna Miller: Hackers, snappers, fashion and me

For a girl whose privacy litigations are legendary, Sienna Miller is compulsively uninhibited. Free, frank and genuinely funny, she seems fascinated by everything - even the sweaty teenage audience of GCSE Poetry Live. We're sneaking round the back of the mega-auditorium of the theatre where she's rehearsing the Terence Rattigan classic 'Flare Path' (which opens at the Theatre Royal Haymarket this week) in search of a room to chat in, but Miller, who is on a lunch break, can't resist peeking in at the kids. 'Bliss!' she stage-whispers. 'You just know the boys are sitting at the back so they can look at the girls.'

Those boys are facing the wrong way today. At 29, the actress who first became famous for being Jude Law's girlfriend exudes coltish, nervy charm. She once boarded at Heathfield School in Ascot, and her speech is still animated by ecstatic girls'-school slang (poetry is 'bliss'; being an aunt to sister Savannah's children is 'just heaven'). But she has grown up a lot in the eight years since fashion editors fell for her floaty, nonchalant beauty and hippy chic, and hailed her as the new Kate Moss. Model-turned-actress doesn't apply any more: Rattigan with Sir Trevor Nunn is the latest in a string of credible, artsy parts ranging from Andy Warhol's self-destructive muse Edie Sedgwick in 'Factory Girl' to Dylan Thomas's wife in 'The Edge of Love'. She even made her Broadway debut in October 2009, playing a posh, masochistic nut-job alongside her 'great friend' Jonny Lee Miller in Patrick Marber's 'After Miss Julie' in a performance a critic at Time Out New York described as 'precise and emotionally raw'.

'That makes one of him!' she giggles, wincing as she recalls other less complimentary verdicts (The New York Times preferred her legs to her acting). Broadway was tough, but an education. 'They're so vocal! Julie's detestable, but you get protective of the people you play. I wanted to make her vulnerable, but I could hear the audience saying, “Yuck, she's disgusting.” It was liberating to have to relinquish my attachment to what they thought. By the end of the run I was just thrashing around on the floor and spitting everywhere.'

Broadway turned her into a theatre junkie: something she describes with near-orgasmic relish. 'It's so weird that this is what I get a kick out of. But there are times when you come off stage and it's so intense that you're screaming and sobbing but you feel amazing. I just don't think you ever feel more alive - it's electric.' She's already planning another play in London in September (details under wraps). 'I'm hooked. Doing a film is just not so appealing.'

When I ask if 2009's 'GI Joe', the mega-money action flick in which she looked so uncomfortable, has put her off Hollywood, her voice shrinks to a whisper. 'Slightly,' she confesses. 'Slightly, yes. It was what it was, but I compromised my own values to a certain degree and I didn't feel good about that.' She looks very small for a second - then her irrepressible sense of fun reasserts itself and she's off. 'I'm not very good at fighting. And I'm not very good at guns. And I'm not particularly aggressive.' Her straight face cracks. 'I had to wear sunglasses,' she hoots, 'because every time I shot my gun I would blink. I was completely out of place, like this little midget running round in black leather winning fights, which was just absurd. They did a wondrous job with the wardrobe to make me look threatening. The director said, “Gawd, you're real short.” I was, like, “Yep, not my problem.” '
'GI Joe' was, at least, 'a great pay cheque. If, financially, I'm okay - which luckily I am - there's no point in doing stuff that's not right for me. It's too much time away from home.' Her next movie project will be more leftfield: controversial Oscar-nominated French-Algerian director Rachid Bouchareb has written 'this thing for me to do which is like “Thelma and Louise” between an American and a Muslim woman. I have to learn to belly dance, which is a complete nightmare. I am woefully uncoordinated.'

Miller is - charmingly and often wilfully - full of contradictions. She pours scorn on the media for portraying her, as the judgmental cliché goes, as a party girl, talking wistfully about wanting 'to live in the country with animals, being quite quiet' (this despite rumours she ended it with Jude Law because his lifestyle was too sedate). But then she raves about The Box, Simon Hammerstein's hedonistic New York Theatre of Varieties which has just opened in Soho - and is anything but quiet. Her sentimental side comes to the fore once more when she talks about nephew Moses. 'I'm auntie Gee Gee. My family call me Gizmo and my nephew couldn't say it. It's the best nickname I've ever had, because one of the people I love most in the world gave it to me.'

Home, these days, is north-west London. And it's a home without Jude Law in it: although she dumped him very publicly in 2005 after he had an affair with his children's nanny, the two got back together when both were starring on Broadway, but ended their relationship again this January. The tabloids, which remain obsessed by Miller and frustrated by her habit of sticking up for her privacy in court, instantly cranked up the rumour mill - linking her with young British actor Joe Armstrong, who just happens to be her co-star in 'Flare Path'…

'I'm single!' she exclaims. 'Poor Joe is, like, “What's going on?” We all find it ridiculous and laughable.' The laughter that bubbles up when she's talking about any other subject, however, is totally quenched by this one. 'I do feel very picked apart by it,' she admits. 'When two people have separated and have both stated that it's amicable and mutual yet, regardless of that… It's not a nice feeling to have people talk about you behind your back and for it not to be true and for you not to be able to retaliate. But it's in newspapers I don't read. And, frankly, c'est la vie.'

A genuinely amicable break-up is a rare thing. But whatever you think about her claims, it's hard not to feel sorry for someone with such liveliness when she looks, suddenly, so tired, wary and sad beneath her golden suntan. There's a widespread and questionable assumption that paparazzi packs and endless bitchy tabloid gossip - multiplied 100-fold in the era of tweets, blogs and message-boards - are a necessary part of the fame game. Miller's clearly hyper-aware that celebrities are envied, and determined not to whinge about being one. But, unlike other actresses, who retreat into a gated life or dutifully smile for the cameras, she has refused to stop going out or to come quietly: in 2008 she won a landmark decision (and £53,000 damages) against leading pap agency Big Pictures, which had pursued her to places where she had, according to the judge, a reasonable expectation of privacy.

'I think I am the most litigious person in this city!' she jokes. 'Some people are fine with that level of attention. And the media had always portrayed me as someone who wanted it. Which was completely unjust, but it sold papers for them.' Put like that, it sounds nakedly misogynistic, I suggest. 'Yes! It's a very sexist world. I just couldn't live my life with that level of harassment. I just couldn't. So it was an opportunity to either stand up for myself and change things or just become an agoraphobic. And yes, I realise that fame has consequences and I want to be gracious about certain things. But this was about standing up for what I regard as a human right.'

When I ask her if she felt she couldn't go out without photographers appearing, she blinks: 'Well, I couldn't go out without them appearing.' How did she stay sane? With difficulty. 'After a while, it makes you crazy. I was completely paranoid all the time, for good reason. And it's aggressive. It was a girl on her own at midnight walking her dog and ten men chasing her down the street. And because they have cameras that's all right. But take away the cameras and what do you have? You've got ten men chasing you down the street in the dark, and it's terrifying.'

Miller has an even bigger lawsuit pending: when she (and others including Steve Coogan) recently filed suits against the News of the World, which they allege hacked into their phones, it precipitated action by the CPS. It also contributed to the ongoing fallout from claims about phone-hacking at the paper - which led to David Cameron's director of communications, Andy Coulson, formerly the tabloid's editor, resigning. 'I don't think it's nice for anyone to lose their job,' says Miller, 'and I'm not allowed to talk about the case. My decision to handle it in the way that I have was just about standing up for myself. But it's quite scary.'

It sounds corny to call someone a free spirit - a bit '70s barefoot organic. And after all, nobody likes being spied on. But Miller's hatred of restriction - by paps, prying papers or anything at all - is physically palpable. She can't sit still. And when she talks about 'being very rebellious about being penned in', she squirms like Houdini. Maybe she needs the protection precisely because she's too uninhibited and impulsive to draw the line between public and private on her own person. (She denies squirting wee at photographers with a water pistol, though she does admit to 'root vegetables', because 'you can't exactly say “She threw a potato at me” in court.') There's certainly no sign that she has cultivated the kind of official personality that media-trained celebs rely on. And when I make the not wholly unreasonable suggestion that the label she helps elder sister Savannah with, Twenty8Twelve, makes her a brand as well as a person, she's outraged. 'Savannah has a first in design from Saint Martins. I don't see it as a celebrity brand. It's quite low-key. It's not like I'm doing a fragrance or anything.' So would she ever do Eau de Sienna? 'No!' But then the straight face cracks again. 'Well, never say…'

Miller's not the only British movie star working for peanuts on West End stages this season: pals Keira Knightley and Jonny Lee Miller are pulling crowds to the Comedy Theatre and the National. She went to see Knightley last week and realised 'that you don't need to do as much as you feel you should. In a space with 850 people and a black abyss, you feel like you've got to be big to reach the back. But it's extraordinary how little you have to do.' As for Jonny Lee Miller's double turn with Benedict Cumberbatch as Frankenstein and the Creature: 'I can't wait! I've seen him with his body - No! Not his whole body, obviously - but you know, he's had to shave everything.'

Sienna Miller's ex, Jude Law, is also back on the boards with trusted 'Hamlet' director Michael Grandage in August ('Anna Christie' is at the Donmar Warehouse). But her decision to join the Rattigan centenary celebrations in 'Flare Path' was less to do with friends and family and more to do with one-man institution Trevor Nunn, who's directing a four-show season at the Haymarket. 'Trevor is, obviously, a box I've always dreamed of ticking,' she says, then claps her hand over her mouth: 'Oh no! Don't put that in!'

In 'Flare Path', Miller plays Patricia, a woman torn between her husband, bomber pilot Teddy (Harry Hadden-Paton), and Hollywood heartthrob Peter (James Purefoy). Rattigan's popular classic is like a wartime 'Brief Encounter' - with extra comedy, much of it supplied by Sheridan Smith's character, barmaid Doris. 'She's terrifyingly brilliant,' says Miller. 'I'll tell you now, she's stolen the show.'

Nunn, who has no difficulty getting big-name talents like Kevin Spacey and Ian McKellen to work with him, asked her to do it. 'He's so gracious. He came to see both my plays ['After Miss Julie' and 2005's West End outing 'As You Like It'] and wrote me letters afterwards, which no one does - he's very proper.' Working with Nunn has convinced her he 'is one of the greatest directors alive. He's so bright. So precise: you feel like you're becoming cleverer just by working with him.' She's nervous, though: 'I didn't go to drama school, and as a result I'm quite insecure about how to approach a script. For me it's more about instinct and blind faith and, thank God, Trevor is the one at the bottom ready to catch the fall!'

The romance and stiff upper lip-heroism of the 1940s has enduring appeal - and will soon pass out of living memory. The 'Flare Path' cast were all choking back tears after a visit from some of the original Wellington Bomber pilots - who have never had a monument erected to them because of shame and embarrassment over Britain's bombing of cities such as Dresden. 'These boys were 18 years old,' says Miller, obviously moved, 'They'd write their last letters every night before going out; they'd get back; there would be empty beds - and someone would collect the letters. I asked, “Were you not terrified?” And they said, “No, we just did it. As soon as you let fear in you'd die.” '

The conflict between duty and happiness is a world away from our time, where it sometimes seems that personal happiness is the only duty. 'I think there's a human instinct to be selfish, especially in matters of the heart,' reflects Miller. 'But there was an innocence in that era that's been lost.' If she'd lived back then, she would have fancied being a land girl. 'Digging for victory in Hyde Park: I love the idea of allotments popping up everywhere.' So fresh air and freedom - that's what she craves. That, and some measure of approval. 'Is that all right?' she asks, at the end of the interview. Then, part-impish, part-pleading, 'Please give us a nice review.' With nerve like that it's not surprising her career's got legs - and hopefully this time round, the critics won't praise them more than her performance.