Keira Knightley's a little peeved. It's the mention of her appearance on The Graham Norton Show, in which she revealed her husband's attempts to teach her the guitar almost ended in divorce, that's done it.

At the mention of his name, the thus far animated and at times gurning Knightley becomes a little steelier of eye, momentarily crossing an arm protectively across her body.

"It was a joke but now I have to repeat it all f*****g day. I do, don't I!" exclaims the actress, who wed the Klaxons' James Righton in 2013.

She thankfully begins laughing (albeit a little hysterically).

She's right though - she probably will be batting off questions relating to her relationship until sundown, but after 20 years in the business, the 29-year-old knows only too well how the promotional tour works. The anecdotes and personal titbits that can be gleaned (however tenuously on the journalist's part) is all part of the circus.

As is Knightley's ensemble - a beautiful black dress with intricate lace and ruffled collar detailing, despite it only being 10am. "It's very swooshy, isn't it?" she says, wafting it about for effect.

Known for eschewing the celebrity circuit despite her Hollywood status, today's outfit is a world away from Knightley's off-duty attire, and that of her singer-songwriter character Greta's (hence the guitar lessons) in new movie Begin Again.

The film's writer and director John Carney (whose 2006 film Once won the Best Original Song Oscar, with the stage adaptation scooping multiple Tony Awards) wanted Knightley's entire wardrobe to come from second hand shops. For Knightley, whose known for her period roles in Pirates Of The Caribbean, The Duchess and Anna Karenina, ditching the corsets was indeed refreshing, but wasn't her sole reason for signing up.

"I wanted something that had hope in it. All the things I'd previously done didn't have hope in them," she says, laughing. "I think hope is a very difficult thing to put into films actually, without being hugely cheesy."

She believes her breakout project, 2002's Bend It Like Beckham, "was a very hopeful piece", though, and that Begin Again shares a similar tone. "I'm not the biggest fan of romantic comedies, but with this one, I thought it was acerbic enough, had dark elements and didn't go too cheesy for me."

The film follows Greta and her long-time boyfriend Dave (Maroon 5's Adam Levine) who, seduced by dreams of making it in the big city, move to New York to pursue their passion for music.

"We all felt it was really important for the role of Dave to be played by somebody who was actually a musician, because they have this confidence that you absolutely can't fake. Actors have been through way too much rejection to pull that one off," says Knightley, who was nominated for an Oscar for her role in Pride & Prejudice.

Despite it being Levine's acting debut, she didn't offer him any advice. "No, he didn't need any, and I don't think you could guide Adam, he's sort of like a missile."

When Dave rejects Greta for the fame and fortune of a big solo contract, she's heartbroken and contemplates leaving the Big Apple. But not before Steve, her mate from home (James Corden) encourages her to go on stage at an open mic night. In the audience is Dan (Mark Ruffalo), a down-on-his luck record producer who, captivated by her talent, persuades her to take a fresh approach and they transform the streets of New York into their recording studio.

"Anywhere else in the world, people are a little curious and quite excited [about a film crew], but generally, if you say, 'Can you get out of the shot?', they'll go, 'Yeah, absolutely!'" Knightley explains.

"In New York, if you say that, they're like, 'No! Get out of the sidewalk!' So I think that was the biggest challenge, to get out of the way before people started shouting at us."

A lot of the movie was improvised ("James Corden is the master of that so he did certainly throw a load of curveballs at me, which was a lot of fun") and Knightley thinks it sums up the "vibe" of the film.

"It's very spontaneous and a lot of the way we were filming was just to go around in vans and try and jump out and film a scene as quickly as possible before anyone noticed."

Although no stranger to challenging herself on screen, she admits to feeling "pretty vulnerable" at the thought of singing on camera.

"We recorded it all before [we started filming], but they hadn't really finished writing the lyrics or the top lines until two days before we got in the studio, so it was very much 'just get on with it' and we had four days to record it all."

That didn't sit particularly well with someone who likes a lot of preparation time.

"I was like, "I have no f***ing idea what I'm doing!" So we did many, many takes of many different styles until we found something that everybody just went, 'Oh! That was it', and it felt right for me as well."

She jokes that the studio experience itself didn't live up to the hype. "I heard all of this stuff that you're supposed to be drunk and have a great time at the studio - all I got was a peppermint tea! I was like, 'This wasn't the idea! What the hell?'

She thinks the film's a fairly true to life portrayal of what happens to couples when the playing field stops being level, and one of them takes off. What happens to intimacy, trust and loyalty when fame comes calling?

"I think the relationship is really well-written. John probably wrote it based on some real-life people he'd witnessed," says Knightley, carefully side-stepping a question as to whether she can relate to such a scenario. "I think there's an incredible excitement that comes from that sudden flash of success and, probably, there are a lot of people that are left in the wake of it, and certainly my character is one of those people."

But unlike Greta, Knightley found fame at a young age. "Being a very introverted 18- year-old girl meant it was a very different experience," she says. "It wasn't like having years of going, 'Why isn't anybody taking notice, why don't they realise?' It was more a sort of, 'Oh f***, that's just happened, and how do I deal with that? I can't go outside any more!'"

As strange as it must have been, she quickly adds: "Luckily, it was so long ago, I don't remember. But I'm still alive, I'm still here and I'm not a drug addict, so I must have been all right!"

She'll next appear in the comedy Laggies with Chloe Grace Moretz and Sam Rockwell, the Alan Turing drama The Imitation Game with "my very good friend" Benedict Cumberbatch, and she "just did a tiny little role in a film called Everest, which looks extraordinary".

She cares little about whether she's the star of the film or merely making a cameo.

"I like to change it up, so if there's a role that interests me, it doesn't matter whether it's a lead, supporting or just something tiny. If I go, 'Oh that's interesting, maybe I can do something with that', then I'll take it."


:: John Carney drew upon his own past as a professional musician to flesh out the character of Dan: "What happens when the jaded music executive of the Nineties meets the optimistic girl from now who can record and mix her album on her computer?"

:: James Corden was starring on Broadway during the shoot for Begin Again and recalls Mark Ruffalo offering him a cheeky G&T disguised in a coffee cup on set to keep him going.

:: Catherine Keener stars in the movie as Dan's ex-wife, while singer CeeLo Green pops up as rapper Troublegum.

:: Following the success of the stage adaptation of Once, Carney's not averse to the same happening to Begin Again but jokes that it would depend on the "money men".

:: Oscar-nominee Hailee Steinfeld plays Dan's rebellious daughter Violet and learned how to play guitar for the role.

Begin Again is released in cinemas on Friday, July 11