Sienna Miller is only inches away from me in a swanky Knightsbridge hotel, but it’s almost impossible to see her. For there before the immaculately presented face of this 26-year-old actress and aspiring megastar sit a hundred headlines, culled from a fortnight of pure media frenzy, that have followed her public split with her boyfriend Rhys Ifans. “Miller Dumps Ifans by Phone!”; “Ifans Cries Onset!”; “Sienna has New Man in New Movie Co-Star!”; “Ifans Threatens to Beat up New Man!” That sort of thing.

Miller is here to promote her new movie, The Edge of Love, in which she gamely stars as Caitlin MacNamara, the tempestuous, rum-sodden wife of Dylan Thomas. She is here to talk about her choices as an actress and her ever-evolving screen career. And she is here to talk about life, her childhood and the experiences that made her who she is today. Unfortunately, everything we discuss, no matter how innocuous, seems to point us back to Ifans-gate.

Her co-star in The Edge of Love, for instance, Matthew Rhys (he plays Thomas) has been insinuated in the break-up. Her last role, in the movie Interview, was that of an actress more famous for her fractious relationships than her work. While her entire movie career was launched, some might say, by her much-publicised break-up with a previous boyfriend, Jude Law. Thus, with all the good will in the world, Miller’s break-up isn’t just the elephant in the room, it is the room itself.

Miller herself is alert, and sitting pixie-like, shoeless and cross-legged, on a footstool in front of my chair. There is another perfectly serviceable armchair on the other side of the coffee table, but this gesture, I can tell, is very Sienna. During our conversation she will occasionally touch my knee (“See, I told you that women were more tactile than men!”), she will throw her head back with laughter, and she will reach out and give me a plaintive look that would break the heart of any man, while saying: “Just look after me in this. Please.”

But for now she is consumed by the Ifans conundrum. “It’s everywhere,” she says. “I try not to read it, but it’s hard not to. I don't want to go, ‘Woe is me in suffering’, but it’s not easy for me.”

Might it be easier, then, to get it off her chest? Can she clarify her relationship status right now? “I’m sorry, but I can’t,” she says. “I have this habit, I’m such a people pleaser, and out of insecurity I think that all people care about is my private life. They don’t want to read about my opinions of film or f***ing politics. I feel like I’ve indulged that need for approval for too long. It's no one’s business any more. It’s too hurtful. I have to learn to censor myself.”

She launches into a lengthy explanation of what it’s like to be “torn to pieces for the pleasure of others. People wouldn’t understand, and I wouldn’t expect them to, what this amount of press attention is like”.

And yet surely she courted this same attention initially? Surely the constant tabloid shots of her out in Primrose Hill, at polo matches, with Sean Penn, with Orlando Bloom, helped to pave the way for her screen career? “It was never intentional,” she says. “It started with my relationship with Jude. Yes, it increases your visibility. Yes, being on the cover of a magazine appeals to studios. But it plays against your work. A lot of people still feel I’m not a proper actress.”

And there’s the rub. Her role as Caitlin tops a hat-trick of performances, following Interview and the Edie Sedgwick movie Factory Girl, in which Miller has demonstrated some serious actorly chops. The true miracle of The Edge of Love is that she engenders sympathy for Caitlin despite her abrasive persona.

Miller says that she feels more secure on camera nowadays, and that her Edge of Love director John Maybury and her friend and co-star Knightley made it easy for her. Of her compelling and vaguely Sapphic screen relationship with the latter (they share baths, beds and strange off-kilter stares), she says: “I think people were hoping for a lot more that you get. I think that Caitlin was quite a sexual person, and I know that she did, well, dabble a bit . And besides, female friendships can often be tactile and intimate.”

We stay, surprisingly, on hot girl-on-girl action for a moment, while Miller remembers her experimental adolescence at Heathfield boarding school for girls, in Ascot. “It was a great school, and it made me learn how to share and to understand women. We’d have the odd snog together,” she says, casually creating a mental picture of St Trinians-style smut. “It wasn’t all the time, but, you know, who hasn’t dabbled?”

Miller left Heathfield for New York at 18, where she studied at the Lee Strasberg Theatre Institute. Her father was an American investment banker, her mother a drama enthusiast who helped to establish the Lee Strasberg School in London. They had divorced when Miller was 6, but her childhood with her mother and older sister Savannah was happy. “There were moments of darkness,” she says. “But nothing that I carry with me today.” Acting, she says, was everything to her, and although she modelled during her drama school days, she dismisses it as “a way to earn money for fees. Model-turned-actress is a terrible tag. I hate it.”

At first the acting roles were sparse. She played a model in four episodes of the Sheila Hancock sitcom Bedtime, and Daniel Craig’s lust object in Layer Cake, a part that she now admits was “gratuitous, just me in my undies”. When she reprised the role, however, opposite Jude Law in Alfie, everything changed. The two became a couple, a fixture in the celebrity pages, and her profile rocketed. Directors such as Lasse Hallström came calling (he cast her opposite Heath Ledger in Casanova), and she beat Katie Holmes to the coveted role of Edie Sedgwick in Factory Girl. She’ll next be seen as a kick-ass action villain in the blockbuster GI Joe, and opposite her Edge of Love co-star Cillian Murphy in the Sixties-set Hippie Hippie Shake. She has also just signed to play Maid Marian opposite Russell Crowe in Ridley Scott’s Robin Hood love triangle movie, Nottingham. “Maid Marian!” she says, shaking her head. “I’m living the dream!”

Despite the current hysteria, her life is “exquisite. I’m not victimising myself, I’m not this tragic figure. My life is privileged, but it comes at a price. And it’s exactly where I’m supposed to be. I make decisions. I take responsibility for them. And I f*** up.”

She says that she has no future plans other than visiting Savannah’s newborn baby. Oh, and getting rid of the house in Maida Vale she shared with Ifans. “I want to sell that house,” she says. “I don’t like it any more. I’m over it.”

She wants to have children eventually, but is not sure if they should be by her side. “I always imagined that I’d like my child around me, but going to boarding school taught me so much. It would have to depend on the child.”

In the meantime, she says, she’s gunning to write and direct. She has written a short film, and says that her Hippie Hippie Shake director, Beeban Kidron, told her that she was a natural director. “I would love to do it, but I don’t know if I have the balls to,” she says. “Or maybe I do have the balls?”

She ponders this for a moment.

She likes the sound of it: “Yes, I’m going to spend the next five years finding my balls! They’re in there somewhere!”

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