LONDONS INDIAN summer is no fun with hayfever but, fear not, Nurse Keira Knightley is on hand with an emergency pollen plan. “So you’ll need an antihistamine – this one is good. And oh, this is really good for stinging eyes. You rub it on your eyelids and it takes the itch away. I hate the stinging eyes, don’t you? I had really bad hay fever last year for the first time. Shocking. I thought I was going to die it was so horrible.”

The queen of costume drama has never played Florence Nightingale. But should the relevant biopic arise, she can skip the audition.

“Now you’re going straight to Boots after this, aren’t you?

We had an inkling she would turn out to be a sensible sort. As long ago as 2006, we can recall a younger Knightley, then aged 20, responding coolly and politely to an idiotic question at the press conference for Pirates of the Caribbean: Dead Man’s Chest: “Why do people keep asking me if I’m going to have children?” she said. “You’re not asking my male colleagues if they’re going to have children.”

She never discusses her private relationships in public, though she has a warmth and openness that occasionally belies her reticence: Mark Ruffalo, her co-star from John Carney’s incoming Can A Song Save Your Life? is “lovely, just perfect”; Joe Wright, who directed her in Pride and Prejudice and Anna Karenina is “lovely and clever”; James Wrighton, her fiance, is “so lovely”.

She and the Klaxons keyboard player were engaged last May; she’s been threatening a full meringue dress ever since.

It could happen. At 27, having survived any number of speculative headlines about her weight, her boyfriends, and the contents of her bins, Knightley is apt to say “sod it” when the occasion demands. She’s even come full circle on costume drama. In 2004 she pleaded with director John Maybury to cast her in the psychological thriller The Jacket with the words “please rescue me from the corset. Almost a decade later, she’s thrilled to be all hooped up as Anna Karenina in director Wright’s new kinetic adaptation.

“I used to think I was doing something wrong,” she says. “I felt as if I had to keep apologising for the period dramas. – ‘Oops. I’ve made another one. Sorry’ – but that’s what I really love doing. I love the fantasy of the past. The here and now is so concrete. I love the sense of exploring and creating other worlds. I love that you can get lost in the possibilities. And my contemporary films never seem to do as well. So now I think, fuck it, I’m going to do what I love doing.”

Today, sporting a delicate monochrome knit and black maxi skirt, the clothes say dressed up, but the way she curls into her chair – feet up – is decidedly less formal. Her arms are toned and belong to someone who goes to the gym, not someone who eats three Tic Tacs for lunch. And the face, though defined by classic movie star architecture, has gotten more impish over the years. She’s almost always on the verge of a dirty laugh.

“I had a word with Michael Fassbender before we started,” she says of their spanking scenes in David Cronenberg’s criminally overlooked A Dangerous Method. “I said: ‘You lay one finger on me for real and I’ll kill you’.”

Though frequently compared with Celia Johnson, the star of Brief Encounter, you can’t imagine Knightley’s logical spiritual precursor star ever warning off a a co-star in such a manner (or signing up for Freudian BDSM games in the first place). The comparison pleases Knightley; she loves old movies almost as much as she loves old books. She devours the latter, despite being diagnosed with dyslexia at six. She has duly appeared for the American Library Association’s Read initiative.

Unsurprisingly, the seed for Anna Karenina was planted when director Joe Wright saw Knightley reading the Tolstoy novel on the set of Atonement.

“I was sure I had read it at 18 or 19,” she recalls. “I was sure I had it on the set of Pride and Prejudice. But Joe says Atonement, and I’m hopeless at remembering what happened in what year, so he must be right.”

You can understand why Knightley’s formative years must seem like a blur. The daughter of stage actor William Knightley and playwright Sharman Macdonald first demanded an agent at the age of three; she was six when her parents relented. She appeared on TV throughout her childhood until her role as Natalie Portman’s decoy in Star Wars Episode I: The Phantom Menace in 1999 catapulted Knightley into the movieverse. The 2002 sleeper hit Bend it Like Beckham won Knightley a cult audience but, beginning in 2003, the Pirates of the Caribbean series would make her one of the most famous women on the planet.

She did not return for the most recent instalment of Pirates, plumping instead for interesting indie projects London Boulevard, Never Let Me Go, and Seeking a Friend for the End of the World.

“It’s easier to concentrate on performance and what you’re actually supposed to be doing on a smaller film,” she says. “Hollywood films are too big and have too many people running around to allow for quiet or thinking time.”

Anna Karenina is lavish beyond its means. With a curtsey to The Red Shoes, the film condenses an epic prologue into a sweeping ballet of bustling people and changing sets.

“The Red Shoes was very much an influence. When myself and Joe first talked about it, it was very much a straight retelling. We were going to shoot in Russia. We were going to have snow and all that. But that would have been far too expensive. So Joe (Wright) called me up one day saying ‘I’ve got it. Come around.’ And when I arrived at his office it was literally covered in drawings. He talked me through the prologue and the race sequence when the horse falls into the auditorium. And my first response was ‘Oh fuck. We’ll never make this work.’

“But the more I thought about it, the more I thought ‘let’s give it a go’. Because if you’re doing a project like this, from a book that’s been visited so many times, it has to be like this. There’s already an iconic Greta Garbo Anna Karenina. There’s no point doing it unless we’re bringing something entirely new and out there.”

Wright’s Powell and Pressburger-inspired version won’t win everyone over, but for admirers its never less than magic.

“Do you know I haven’t seen it yet?” says Knightley. “I’m dying to know how it turned out. I love Joe’s work. It if had been any other director, I don’t think I would have had the courage to say yes. But there was a team on the film that have all worked together before. And we were bloody determined to make it happen. Even if that meant I had to dance. Which was fucking impossible.”

Has her view of the novel changed since she first read it on the set of Atonement or possibly Pride Prejudice?

“Definitely,” she says. “I remembered the book as being just incredibly romantic with this extraordinary woman at the centre of it. But in re-reading the novel just before we started filming, I found it to be much, much darker. It’s a huge moral quandary. Anna Karenina is a heroine but she’s also an anti-heroine. Tolstoy is obviously a genius and he understands how complicated human beings are. So there are so many layers to Anna. She was the hardest role I’ve ever done. She had to be sympathetic but she couldn’t be too nice. She leaves her son. She does terrible things. But she’s also this amazing, feeling creature who is treated so unfairly.”

So she studied?

“Oh yes. The boys were a bit intimidated when they saw how many post-it notes I had.”

Following on from the 27-year-old’s appositely heightened turn as Sabina Spielrein in A Dangerous Method, Knightley’s Anna is more animated in her distress than famous predecessors Garbo and Tatyana Lukashevich. She does seem to enjoy her crazies, I suggest.

“I do. I love having a lot to think about. For me, that’s what its all about.”

Having essayed Dr Zhivago’s Lara early in her career and now, the tricky Anna Karenina, she must want to check Bovary off the list, surely? “Absolutely. I have to do Bovary now. I’m available.”

She has, at least, spent time away from petticoats and insanity for John Carney’s Can A Song Save Your Life?, the Irish director’s follow-up to Once. The film wrapped earlier this summer and sees Knightley wield a guitar as a singer-songwriter who hooks up with Mark Ruffalo’s jaded record executive.

“In my last three films, I’ve lost a liver, been hit by a train and faced the apocalypse. And along comes this lovely, sunny movie. If anything, it was too sunny. I never knew that New York stays so hot for so long. John Carney and I were in the shade complaining every minute we could get.”

We exchange farewells but we’re not getting away that easily: “You are going to the chemist, aren’t you? Promise or I’ll march you there myself.”

Yes, Nurse Knightley.

* Anna Karenina is out now on general release