When Keira Knightley first found fame, the world was a very different place. "I remember telling my mates I was doing a film based on a Disney theme-park ride," says the 34-year-old actress, thinking back to the moment shortly after her 17th birthday when she was cast in Pirates of the Caribbean: The Curse of the Black Pearl. "And they laughed and went, 'Well, that's your career over then.' Whereas these days, it would seem like the luckiest imaginable break."
Ensconced on a sofa in an elaborate floral-print dress that makes her look like the world's most glamorous scatter cushion, Knightley in person is exactly as you picture her: dark gaze, determined jaw, smile that flicks between conspiratorial and gracious. Or she is right now, at least. It is the day of the London Film Festival premiere of her new political thriller Official Secrets, and she has spent the morning attached to a breast pump in order to make it through the afternoon's activities without a leak. She gave birth to her baby daughter Delilah – a younger sister for three-year-old Edie – seven weeks ago, and has carved out of her schedule six months of maternity leave. But today is a long-planned blip, and she's had to strategise.
"Three whole feeds," she explains, with an audible note of relief. "We're going to be apart for six hours in total, so I pumped three feeds' worth." Her husband, the songwriter and former Klaxons keyboardist James Righton, is at home with the bottles lined up, while a fourth session has been timetabled for after our chat. "If I don't do it my boobs will explode," Knightley explains. "I mean, quite literally. It's going everywhere. So, you know, that's not happening. The pump is with me. It's fine."
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Yes, much has changed since Knightley's Pirates days. Once a famously guarded interviewee, she breezes through our conversation like the chattiest mum at Tumble Tots, who may or may not be amusing herself by trying to make the only dad present blush. The plot of Official Secrets, however, dates from her younger, warier days – and centres on leaks of an altogether dicier kind. The film tells the story of the GCHQ whistleblower Katharine Gun, who made public a memo from the United States' National Security Agency that requested British assistance with an illegal bugging operation designed to sway a forthcoming UN Security Council vote on war in Iraq.
Knightley was unfamiliar with the story when she accepted the role. Both the whistle-blowing itself and Gun's subsequent arrest and trial had unfolded during the Pirates shoot and press tour in 2003 and early 2004. But even her more politically engaged friends could barely recall it having taken place. "Looking back, I think Katharine's story just got swallowed up by the actual invasion of Iraq itself," she says. "Yet now it feels like an important piece of that puzzle. We're still living with the legacy of that war, so trying to understand the lead up to it made this feel like an important story to tell."
Disillusionment with conventional politics is arguably a part of that legacy, she goes on. "I remember when it became clear that there were no weapons of mass destruction, and how that felt. And I think a lot of the apathy of our generation stems from that realisation we had been lied to, and that nothing would be done about it."
The first time we see Knightley's Katharine, it is Wednesday Feb 25 2004: the morning of her trial at the Old Bailey in London. She is about to make her plea – but its full significance only becomes clear in light of the covert dealings and legal wranglings that led her to the dock. It is staged by writer-director Gavin Hood with allusions to the post-Nixon conspiracy thrillers of the Seventies, such as All the President's Men, albeit with a distinctive British edge. There is a lovely scene in which secrets are swapped in an underground car park, and Matt Smith, as the Observer journalist Martin Bright, flinches with embarrassment that it's "all a little Deep Throat".
Knightley met Gun while preparing for the role, in the hope of going over her experiences "with a fine-tooth comb," but this proved uniquely tricky. "She's the first person I've spoken to who legally couldn't answer a lot of that stuff, because she is still bound by the Official Secrets Act." Even so, on screen Knightley nimbly makes sense of her character's complex motivations: she's no sleeper agent, but a woman whose conscience is pricked in a very precise spot. Whether or not you remember how Gun's trial unfolded, it is no spoiler to say that her ultimate aim of preventing the Iraq War didn't pan out.
"It would be lovely to put a Hollywood ending on this film: 'And then the war didn't happen'," Knightley says. "That up to a million people didn't die and a whole region wasn't destabilised. But you can't do that."
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So what did Katharine achieve? "She stood up," Knightley says, after a pause. "And very few of us do. Very few of us actually put ourselves on the line. We try to save ourselves instead."
Knightley and her husband have been wrestling with this lately while watching the Extinction Rebellion climate change demonstrations unfold. They've spoken seriously about joining the battle lines themselves, but she keeps coming up with pragmatic reasons – "excuses, really" – not to get involved.
"We keep having this discussion where we're saying to each other, 'Should we get arrested?'," she explains. "And I'm like, 'We can't, we've got a baby, she's permanently attached to my boob. So how's it going to work?' So then he's going, 'Maybe I'll go down and get arrested for both of us.' And I'm like, 'Let's neither of us get arrested now. It's not the right time.' I think a lot of us just want to find a reason not to think about it."
Knightley's own upbringing was, she says, "very politically inclined": her parents, retired actor Will Knightley and playwright Sharman Macdonald, were members of the Left-wing theatre company 7:84, a hotbed of Seventies agitprop. The group disbanded shortly before Knightley was born, but its principles regularly rang out over the dinner table at the family's three-bed terraced house in the southwest London suburb of Teddington. "They were very definitely on the Left side of the Labour Party," she remembers. "So the discussion in our house around the war was huge." Both Knightley and her older brother Caleb, now a composer and sound designer, were taken on the anti-war marches in London in her breaks from shooting the first Pirates film. Acting lay on the same continuum as activism: "I grew up with this idea that it could be an incredibly powerful political thing," she recalls.
Presumably there was much consternation when she went from her breakthrough role in the coming-of-age comedy Bend it Like Beckham to a Disney blockbuster, then? "It was always meant to be theatre!" she laughs, explaining that she started auditioning at 12, "but didn't get the parts, and then the film thing just sort of happened." Her awareness from childhood of just how parlous the line of work can be made her loath to turn down any opportunity. "I still feel like that. A little space in the world opens up and you have to jump through it. Because it may never come again. So I jumped."
The ensuing six years, from 2003 to 2008, were lunacy: three Pirates films, five period dramas, two thrillers, an all-star romcom (Love, Actually), a famous turkey (King Arthur), an Oscar nomination (for Pride & Prejudice) and a Bafta nod too (for Atonement). There were times that she treasures, such as the making of Atonement – a "magical" summer in Shropshire adapting a book that had been widely written off as unfilmable. ("That's kind of what you want," she says. "Everybody going 'Nah, won't work.' Because then if it actually does, it's amazing.")
Her character Cecilia Tallis's hallucinogenically beautiful green silk georgette dress, designed by Jacqueline Durran, has since been enshrined as one of cinema's all-time-great frocks. Why did it click? "Because it was what she was," she says simply. "Brittle. Narcissistic. If you pulled at it too hard it would just fall apart. But also terribly glamorous, you know."
Did she know it would be a career-defining moment? "I knew it was a f------ good dress," she grins.
But such respites were rare, and the relentless pressure led to her suffering a mental breakdown at the age of 22, which she almost felt she'd been willed into by the photographers that shadowed her every move.
"The value of photographs of any famous young women at the time went up if they were of a very negative nature," she recalls, referring to the frenzies around Amy Winehouse and Britney Spears. "So if you weren't already having a mental breakdown, they were trying to push you into doing things that kept your value as high as those who were." She recalls a fad for pictures of car crashes: "I remember Scarlett Johansson getting forced off the road in Los Angeles, then someone trying to do the same to me in Kentish Town. I told them I was going to kill somebody; they said they'd get more money if I did." Her tone is nonchalant, but you can hear the scar. "I don't know how things are now," she ventures, "but it was definitely a moment I wanted to run away from. It did not feel like it would end anywhere well."
After a two-year hiatus, she returned with a new career plan. "I knew I didn't want to do big-budget films any more, because the fame that came with them I just couldn't handle," she says. Instead, she sought out roles that cut against her "English Rose" persona: favourites include a pensive clone in the Kazuo Ishiguro adaptation Never Let Me Go and a psychiatric patient with a yen for S&M in David Cronenberg's A Dangerous Method. "If I was presented with the script for Pirates of the Caribbean today, I'd be wrong to do so, but I'd probably say no," she says.
She recently rewatched the Godfather trilogy, a favourite since her teens. This time, the films played differently: put it down to stage of life. "The moral corruption that Al Pacino embodies within the second film – I don't think I fully appreciated that when I was younger," she says. As for his performance: "He really does nothing. It's extraordinary." It's a collectedness she's always hoped to bring to her own work – and perhaps will find the right role to do so once the maternity leave has run its course.
"I'm always looking for something that has that utter centred stillness," she says, "And not because somebody just wants you to look pretty."