The hotel where Keira Knightley conducts her interviews might have been built especially for the occasion. It was erected at speed in a vacant lot that was previously home to a collection of dumpsters; a testament to a boomtown London in constant gentrified flux. The stage is hastily dressed with photographers at the kerb. A liveried footman is installed by the door. Upstairs in the suite sits a young movie star. I am tempted to view her as a work in progress as well.

What a hectic and heady career Knightley has had. She has been praised and razed, built up and torn down; variously hailed as the most vibrant British actor of her generation and dismissed as an am-dram fake who got lucky. Hers is an apprenticeship that was played out on camera. It has bounced her from homegrown heartwarmers to Hollywood blockbusters to costume dramas and back again, to the point where she can't quite recall the work she has done or the people she knows. "Oh, hello, we've met before," she exclaims, although I assure her that we haven't. At the tender age of 29, she figures that she must have encountered everyone at least once.

In the past few years we have seen her as a hysterical psychiatric patient in A Dangerous Method and a doomed, tragic aristocrat in Anna Karenina. Today, however, Knightley has brought her long limbs and regal jawline into town to discuss a sunnier guise in a musical of sorts. The film is called Begin Again and that title is so freighted with corny significance that she keeps nodding at the poster, as though it is there as her prompt. She explains that she had arrived at a crossroads, or maybe the end of act one. She was feeling stuck in a rut, mired in neurotic roles, and she longed to break herself free. Knightley rolls her eyes and dissolves with mirth. "It's like beginning," she hoots. "But it's beginning again."

Actually, I'm all for Knightley expanding her repertoire; I just wish she had found herself a sharper playlist. In Begin Again, she stars as Greta, a heartbroken folk singer adrift in New York who helps a rumpled, down-on-his-luck record producer (Mark Ruffalo) up the road to redemption. The film is written and directed by John Carney and it plays like a moneyed, remastered version of his 2007 breakthrough Once. Begin Again lacks jeopardy; it lacks grit. For all their flirty banter and boisterous recording sessions, the characters in Once were barely clinging on by their fingernails. Here, by contrast, they are largely mooching and wallowing, spinning mountains from molehills as the plot shunts them between a series of picturesque Manhattan backdrops.

But I may be approaching the movie from the wrong angle altogether. Knightley, after all, explains that she was drawn to the project precisely because it felt hopeful and positive; because it provided a light and airy extension to her main body of work. Moreover, I'm guessing that its fixation on mixtapes, favourite albums and guilty pleasures struck a chord with the actor, who married the Klaxons keyboardist James Righton in May of last year. For an instant she looks positively stricken. "Oh God, fuck no, I'm so not musical," she splutters. "It's terrible. I know nothing about music whatsoever. I mean, I could probably bullshit you and come up with an album that I've heard is really cool, but you'd probably find me out."

But she shares a house with a pop star. She must be surrounded by music every day of her life.

"I probably am. I just don't notice; I don't even hear it. I've got a lot of friends who are musicians and they give me albums and I furiously march around, trying to listen, but it never sinks in." She racks her brain. "I used to like Rage Against the Machine, but I think that was only because they swore in their songs. I was always more into reading and drama. I was such a geek. Which is quite interesting in terms of memory. There's often a huge link between music and memory. And I've got such a bad memory. That's why I couldn't remember meeting you before."

Again I'm confused: she hasn't met me before. "Well, that's what I mean," she says. "I can't remember a thing."

Perhaps we should recap the recent decades – for her own benefit as much as anyone's. She was raised in suburban west London, the daughter of dramatists, and famously requested an agent before she turned four. In her teens she took a small role in The Phantom Menace, starred in Bend It Like Beckham and picked up an Oscar nomination for her performance as Elizabeth Bennet in Pride and Prejudice. She has acted alongside Johnny Depp in Pirates of the Caribbean, suffered soulfully through Atonement and flogged haircare products and Chanel perfume. But her runaway acclaim made her tabloid catnip as well. She was forced to deny rumours that she suffered from an eating disorder and found herself stalked like a frightened gazelle whenever she set out on foot. "I like walking," she says ruefully. "I know that sounds like a stupid fucking thing. But I do like walking, and for a long time I couldn't."

She probably remembers more than she cares to admit. Success, she allows, arrived a little too soon. "I've definitely done all my learning publicly. And I've had to develop a thick skin because of that. So yeah, there would have been a comfortable way of doing it where I went to drama school and made tons of mistakes. If you could choose how success happens, that's what I would have chosen. But of course you can't. And if a moment comes, you have to jump, because it probably won't come around again. So I chose to jump, knowing it was going to be brutal because I knew that I hadn't learned enough. I didn't know what I was meant to be doing."

The way Knightley tells it, her equivalent of drama school was the set of Hollywood blockbusters and high-end costume sagas. She was feted and fawned over; she treated an Olympic ski jump as her nursery slope. Maybe this accounts for the flickering hostility that she seems to inspire. Few rising actors divide an audience to the extent that Knightley does. Where fans applaud the open, unguarded quality of her screen performances, others see an unformed ingenue playing dress-up in the mirror.

"The criticism was tough," she says. "At school I'd always been good at things. And drama was always something I'd been good at. I was picked for good parts in the school plays and you form a sense of yourself from that. And then all of a sudden people are saying: 'No, you're shit. You don't know what you're doing.' It's a strange jolt in how you see yourself and how you connect to the thing that you love."

There is little that gets up people's noses so much as a young, pretty actor who it is felt (rightly or wrongly) has not quite paid her dues. "Yeah, yeah, and I get that," she nods. "It's really annoying, let's face it. 'Fuck off, you little shit.' I do totally get it."

In her mid-20s, around the time most actors are just getting started, she made a conscious decision to edge back from the limelight; to take on smaller productions and find a place she felt comfortable. But she still seems unsure just how far she should go. Her heart tells her one thing and her head says another. "The majority of my work is still in the commercial field. It hasn't gone hugely experimental, which I would really like to do."

One day, she would love to make a film like Under the Skin, which blew her away; she'd never seen anything like it. She also loved Holy Motors, with its talking white cars, and all those films by Jacques Audiard; she can't remember the names. "This is going so well," she guffaws. "I'm forgetting absolutely everything."

If these are the films she would like to be in, I don't know what's stopping her. Her presence alone may be enough to finance an ambitious, leftfield little movie. Knightley pulls a face; the prospect seems to alarm her. "Yeah, it can do, but then your box office comes back into it," she explains. "You're only useful in financing those films if you've got a big success behind you, and they have a certain shelf life. So your stake in financing those films goes down very quickly. That's why you have to tread a careful line. That's the game we're all playing."

I have the sense this is an issue that is probably best left to another day, another year. Who cares if Knightley does not quite know where she wants to go next? She has some time on her side, she is catching up with herself. And the next item on her agenda isn't a movie at all. Today, it transpires, is her brother's birthday and he is throwing a party. Interview complete, she prepares to brave the photographers on the way to her car. I hope she remembers the address; I hope she's got the date right. A few minutes later, I pass her down in the lobby. "Hello," she laughs. "We've met before."

Begin Again is out now.