When Keira Knightley was small there were two things she knew she would become. First, an actor. That worked out. She got an agent at six; was in Star Wars at 13 and Pirates of the Caribbean at 17; won an Oscar nomination for Pride and Prejudice at 20 and another, 10 years later, for The Imitation Game. Second, a man. That’s still a work in progress.

“I remember everything about that feeling,” she says, now 33, folded up on a sofa in a London hotel. A big blue frock juts out from under her like a nest. “That girls grew into men, and that’s what I was going to be.” Toddler logic, she admits; goodness knows what boys became. “Maybe it was that the girls were the most powerful in the playground. They were in charge and, obviously, the men were in charge outside. So clearly that’s where I was going. Only, of course it wasn’t.”

Knightley scrunches her face: a chipmunk photobombing a supermodel. Still, she was a grade-A tomboy: no skirts, leading a protest at school until girls were allowed to play football. At 11, she was “obsessed by The Godfather. I wanted to be Al Pacino and that’s where I was heading. The great parts are the guys’ parts. You don’t want to be the pretty girl in the corner or the mum being lovable and supportive. Of course, when you grow up you are, but you still want to have the adventures.”

Knightley’s entire career, from ball-busting breakthrough in Bend it Like Beckham to cross-dressing, trans-romancing Colette in her new film, has been an attempt to have adventures her younger self would respect. To explore if not what it would be like to be a man, then certainly “the masculine side of the female, stuck in the dresses and makeup. Almost every character I’ve played has tried to break out of that image of femininity. That’s why I like period films, because it’s such an overt cage you put the woman in. That’s always something I’ve really identified with. I feel like I sit somewhere else.”

I look quizzical. “I’ve never wanted a penis,” she clarifies. “Apart from to piss up a tree. Being able to do that standing up: so convenient. You can just whip it out and whatever. But the idea of something so vulnerable swinging between my legs, I think I’m all right without.”

Generally, it can be tricky encouraging stars away from unpacking art to chatting genitals. With Knightley, it’s like turning on a tap. I don’t think I’ve ever met someone simultaneously so famous and so garrulous (she’s also bracingly sweary). Or perhaps it’s something only those who have been A-listers more than half their life already can get away with.

Watch Keira Knightley’s interview on The Ellen DeGeneres Show

A few days after we meet, Knightley appears on Ellen DeGeneres’s chatshow to promote The Nutcracker and the Four Realms, and mentions she has banned her young daughter, Edie (with husband James Righton, frontman of the new-rave band Klaxons), from watching the Disney classics Cinderella and A Little Mermaid because of their dodgy feminist messages. Twitter entered white-heat mode. Thinkpieces were spewed out like ticker tape. Presumably Disney – which also made The Nutcracker, which has since flopped – weren’t thrilled. But when asked about it again a fortnight later, instead of calming the waters, she went in with a whisk: doubling down on her argument with some PhD-level Disney princess knowledge.

Still, she says in December when we speak again on the phone, being up for a fight doesn’t mean you want a punch. “I thought I was just being perky in an interview. My God, people feel really strongly. Don’t fuck with Cinderella. Her fans will end you.”

Anyway, back to Colette. It’s the latest film from Wash Westmoreland, who made Still Alice with his husband, Richard Glatzer, before the latter died of ALS (also known as motor neurone disease) a fortnight after Julianne Moore’s Oscar win for lead actress. They co-wrote this one, too, and it’s clearly the work of a couple: intimate, barbed and funny. The plot revolves around Colette and her literary entrepreneur husband Willy (Dominic West), who pass off her novels as his, until Colette starts resenting the fiction, and Willy’s hypocritical jealousy over her affairs with a Louisiana heiress (Poldark’s Eleanor Tomlinson) and a gender-fluid aristocrat (Denise Gough) who never removes her suit. By and large, men fare badly. But Willy is, like Colette, finely drawn, charming for all his ludicrosity. In one scene, he wins his wife back by explaining that men are the weaker sex, slaves to their biology, ergo his endless shagging.

Bend It Like Beckham, Knightley’s 2002 ball-busting breakthrough. Photograph: Allstar/BSKYB

“The Weaker Sex” is also the withering title of an essay Knightley wrote for a collection called Feminists Don’t Wear Pink And Other Lies, published in October. At the time, it caused a stink for what some perceived as an attack on the Duchess of Cambridge for looking glam hours after the birth of her daughter. Was she surprised? “The whole essay was about the silencing of women’s experience. So it’s interesting that’s exactly what happened from certain media outlets. They turned a moment of empathy from one body to another around to say: she’s shaming her.”

Yet women seemed just as mean as men. “Yeah, that’s interesting. Internalised misogyny? I’m not criticising that. All of us respond to and survive within the culture in the way that we can. But I think we need to have a big look at ourselves.”

Indeed. The duchess section is actually the blandest part of a blazing manifesto that starts with Knightley’s vagina splitting, swiftly brings in blood, poo, cracked nipples and incontinence pads, then closes with a broadside against male colleagues:

“They tell me what it is to be a woman. Be nice, be supportive, be pretty but not too pretty, be thin but not too thin, be sexy but not too sexy, be successful but not too successful. … But I don’t want to flirt and mother them, flirt and mother, flirt and mother. I don’t want to flirt with you because I don’t want to fuck you, and I don’t want to mother you because I am not your mother. … I just want to work, mate. Is that OK? Talk and be heard, be talked to and listen. Male ego. Stop getting in the way.”

The essay, says Knightley, was “just sort of vomited out”, submitted in a “fuck it” moment, published “because we have to harness this moment in time and use our voices to keep the conversation going. Because we’re saddled with a system built on inequality,” she says, “progress is going to be slow and painful and uncomfortable. But I want to make sure I’m not raising my daughter in fear of the whole other half of the human race. Just as it’s important to raise boys seeing the whole of a woman’s experience, not simply one aspect of femininity. Otherwise, how can you respect it?”

With Denise Gough in Colette (2018). Photograph: Allstar/Number 9 Films

The core problem, she reckons, is that the voices of a generation of women are lost to childcare. This means – to bring it back to straying Willy – that “we all empathise with men hugely because, culturally, their experience is so explored. We know so many aspects of even male sexuality. But we don’t feel like men can say: ‘Yes, I understand what you’re talking about because I’ve got this wealth of art and film and theatre and TV from your point of view.’”

So if women don’t come on flirty or maternal, she thinks, some men get stumped. “Before motherhood, you’re sexy, but if we talk about the whole vagina-splitting thing then that’s terrifying; there’s no sex there, so what we do is go into the virgin-mother retrofit, that’s nice and safe. The problem with those two images is I think very few women actually identify with them. Women are meant to play the flirt or the mother in order to get their voice heard. I can’t. It makes me feel sick.””

On set, she was informed by a male director – not Westmoreland – exactly what she was: not passive-aggressive, but openly aggressive. Her eyes pop recalling the shock. “I thought that was extraordinary. I hadn’t raised my voice, I hadn’t sworn, I simply disagreed with a point. And this was someone I liked.”

Look back over Knightley’s previous press and what sticks out – more, even, than her habit of irritating people by dint of crimes including her looks and niceness – is her composure. She has always seemed poised, someone of conviction and confidence, if not arrogance. Bumblings have been low-key; she has kept it together.

Knightley in A Dangerous Method, 2011. Photograph: Allstar/Lionsgate/Sportsphoto Ltd/Allstar

Not without effort, it turns out. Earlier this year, she told the Hollywood Reporter that she had had a breakdown aged 22, following five years of endless exposure. She didn’t leave the house for three months. A therapist said she was her first client who wasn’t being paranoid when she worried about people following her. She needed hypnotherapy to feel able to walk the Baftas red carpet for Atonement in 2007.

She switched focus: a year off, then smaller, odder films with more difficult heroines – The Edge of Love, Never Let Me Go, A Dangerous Method. She upped the robustness of her engagement with the press, suing the Daily Mail after it implied she was a poster girl for anorexia. Today, she credits that crisis with being able to handle the first few months of motherhood. “Your body just created life and now it’s shifting in order to feed it. That’s monumental and we’re all expected to go: ‘Oh no, all good, I’m groovy – I haven’t slept, I’m fine. That I’m able to forgive myself for not being brilliant [as a parent] every fucking day is probably because of that breakdown.”

And that is another reason why marginalising women once they have had children is dangerous, she says, if not warming to her theme (she never really strays from it) then expanding her thesis. It means this sort of experience is sanitised and so those struggling feel even worse. She read recently that 50% of new mothers have mental health difficulties. “With children, it’s one in 10 and that’s called a crisis. So what’s one in two? That’s a fucking catastrophe.

How, as a society, are we not supporting single mothers 100%? We should literally be wrapping them in cotton wool and giving them a cuddle

“We have to talk about it so we know we haven’t failed. It’s really difficult for me, who has an unbelievably supportive family and the money to pay for good childcare. How, as a society, are we not supporting single mothers 100%? We should literally be wrapping them in cotton wool and giving them a cuddle. Saying absolutely we will help [them] as much as we possibly can. That we’re not seems insane.”

The other reason Knightley speaks her mind is it would never occur to her not to. The perception is that she hails from high privilege. But her schooling was bog-standard; her parents – playwright Sharman Macdonald and actor Will Knightley – thespy, but also relatively radical leftwing types. Not quite hippies, says Knightley, but as close as Teddington in south-west London probably got. She says she hadn’t realised how lucky she was to be raised by a father who loved the fact his wife often earned more than he did.

In the film she is currently shooting, Misbehaviour, she plays Sally Alexander, a women’s libber among those who invaded the stage at Miss World 1970 with football rattles and flour bombs. Watch footage of the real Alexander today and you could be watching an older Knightley. These are her people: arty but determined, polite but forthright.

And that, perhaps, is why Knightley’s pronouncements don’t – for me at least – stick in the throat. So much political rhetoric trotted out by film stars smacks of the bandwagon: rote homilies, recited without too much consideration.

Knightley with her husband James Righton and her parents Sharman and William, picking up her OBE, December 2018. Photograph: WPA Pool/Getty Images

Knightley sings from a more sophisticated hymn sheet. Take Miss World. Plainly, she’s not a fan: “I’d never seen a beauty competition before but you think, wow this is really out there! The way they turn their arses and the cameras panning up and down. The way society as a whole goes: ‘Oh no, this is fine.’”

But while the actions of Alexander et al were “amazingly brave”, she’s not sure she would have done the same, because the contestants felt they were being attacked. “That was unfair. And having been on stage, if something like that happened it would be utterly terrifying.”

She is even measured when I asked about Harvey Weinstein, giving “credit where credit’s due; he was very good at getting independent cinema an audience”. They had most contact around the time of The Imitation Game, when she wasn’t remotely in need of patronage, and pregnant – both reasons he didn’t try it on, she thinks. “And maybe he just didn’t fancy me, could be as simple as that.”

With Harvey Weinstein in 2014. Photograph: Mark Davis/Getty Images

She swears ignorance over his alleged assaults. “I absolutely knew he was a womaniser, because you could see it. But I thought that was consensual, and I’d never heard he’d raped anybody. Everyone knew he was a bully because he would scream and shout. But it wasn’t obvious he was doing what he was doing with the bathrobe and the massage and the pot plant. The pot plant!”

It is rare that Weinstein is discussed by anyone with clear eyes and a not totally po-face. Does she worry the debate triggered by the revelations about his alleged behaviour is misandrist? “Absolutely. There’s a time [women] should be standing up and howling and making as much noise as possible, saying: ‘Hey, this system doesn’t work for one half of us.’ But that it makes it very difficult for men to speak and I think there may be some things we don’t want to hear.”

Such as? “About their sexuality, how they see us, exactly what they want. It’s a really tricky discussion but I don’t know how we move forward without engaging men. And you can’t hate them if you’re trying to do that.” A pause, another grimace: scepticism made flesh. “Also: they’re nice. I know some really lovely ones.”

• Colette is released on 9 January.