Think of all the clothes that are thrown out every day. The sock that sprung a hole in the toe. The trousers that shrunk in the dryer. The daring top you bought on a whim and never summoned the confidence to wear. According to a 2017 survey, Britons throw out 300,000 tonnes of clothes a year, representing an annual value of around £12.5bn – which is close to the £350m a week Boris Johnson once pledged to the NHS in Brexit savings. That’s a lot of nurses.

Now imagine you had chosen to donate your clothes to Oxfam. Instead of polluting the oceans and adding to landfill, they might be gracing the actor Naomie Harris in a new campaign for the charity. On a wet October afternoon, she is cycling through a sequence of poses in a New York photo studio overlooking the Hudson River. A woman from the charity tells me they chose Harris “because she’s fabulous” and for once the word does not seem like hyperbole. Dressed in faux fur, her loose hair rippling in the current of a wind machine, Harris looks resplendent as she flings out her arms to Shakira’s Hips Don’t Lie. Behind her, a young man in a Mickey Mouse T-shirt clutches one end of a gauzy orange canvas and jumps up and down in order to make the material flutter prettily in the background.

Although vintage fashion has always carried a frisson of cool, telling someone they look like they shop at Oxfam is not generally regarded as a compliment. Harris is hoping to change that by partnering with the charity to promote secondhand clothes as the best and least compromised way to support sustainable fashion. It’s a stance that comes easily to her.

“I’m the hand-me-down kid, so I was never ever interested in clothes at all,” she says. “I was happy to get my cousin’s hand-me-downs and they fit me. They kept me warm and that’s all I cared about.” This turns out to be one of many very un-actorly things about Harris, who seems remarkably self-contained and self-sufficient and uncontrived. When she talks about fashion and sustainability there’s no sense in which she’s just posing as an evangelist. She reels off a series of damning facts and figures to shame anyone who has ever succumbed to one of those wretched ads on Instagram for bamboo fibre pants or cactus silk-screen gym shorts (guilty on both counts). “I’m not perfect by any stretch of the imagination and I don’t expect everybody else to be, but you can go to Oxfam, find a load of clothes for a night out, absolutely,” she says.

‘I’d like to live off-grid, solar panels, rainwater, growing my own veg’: Naomie Harris. Photograph: Mamadi Doumbouya/The Observer

Harris says she has “never really been very interested in appearances”. Early in her career she didn’t even dress up for the red carpet. “I just wanted to be myself,” she recalls. “Sometimes I wore jeans and a T-shirt and some trainers, and I thought I was so cool.” By contrast her mum, who spent 13 years writing for EastEnders, was the glamourpuss. “She’s always got the lipstick on, she loves clothes,” says Harris. “I was just born opposite to that, for whatever reason.” As a consequence, she is not possessive about clothes. Asked to nominate a favourite item from childhood, she shrugs in defeat. “I never got attached to things,” she says. “I’m still like that today.”

Was she rebellious as a child? “No, not at all,” she says, with an emphatic shake of her head. “I was a rule follower.” She attributes her discipline to her mother. “It’s a very interesting way to raise a child, which is that you don’t set boundaries, and so they self-police,” she says. “There wasn’t anything to rebel against.”

It may be a wet, dreary afternoon, but in the studio the atmosphere is buoyant. Harris has been awake since 4.45am, and has already packed in several interviews as well as the photoshoot. Her energy is unflagging, a quality she attributes to several daily sessions of meditation and a strict bedtime policy.

It is a day after the premiere of her new movie, the shrewdly topical Black and Blue that grapples with the challenges of being a black cop in America, but Harris had skipped the after-party to ensure she got her requisite shut-eye. “I’m quite militant about getting to bed before 10pm, and I’ll leave a party if necessary,” she says. “I don’t drink, I don’t smoke, and I only really drink water.”

Arresting drama: Naomie Harris with Tyrese Gibson in Black and Blue. Photograph: Alan Markfield/AP

If this doesn’t sound like most actors you’ve ever read about, Harris is not like most actors. She flinches at the suggestion that some might consider her – horror – a celebrity. “I was never that person who was on covers of Hello!, telling stories about my personal life,” she says. “I’ve never been a celebrity as such.” Until Black and Blue, she consistently turned down offers for lead roles because she felt the weight of responsibility too acutely. “I couldn’t imagine having the pressure of actually carrying an entire movie and then feeling responsible for its performance at the box office on top of that,” she says.

Harris does not tend to go in for deep bonding with other actors on set (perhaps a given when you only drink water), preferring to bring along her mother and 23-year-old brother for the ride. “You’re like a gypsy in this profession, constantly travelling around on your own, and rootless, and meeting all these new people,” she says. “So, when you get some kind of continuity by bringing your family with you, it’s really very special.”

Ever since her breakout role in Danny Boyle’s 2002 zombies-on-acid flick, 28 Days Later, Harris has managed the remarkable feat of landing roles in crowd pleasing franchises such as Pirates of the Caribbean and Bond while maintaining her fiercely guarded privacy.

She did, however, recently add to the mountain of #MeToo stories during an interview with the Guardian, recalling an audition during which a “huge star” put his hand up her skirt in plain view of the director and producer. “I was in my 20s, and felt totally powerless to say anything,” she says now. Suffice it to say, she didn’t take the role. But she knows from talking to others that she’s lucky to have escaped worse. “I’ve always had phenomenal working experiences and worked with really, really lovely people,” she says. “And I don’t know how that’s happened, because I’ve spoken to a lot of other actors and they’ve had hideous experiences on set, and worked with real bullies. I’ve always managed to escape that somehow.”

‘It was this low-budget movie that I did for a minimum fee and it’s had a phenomenal impact’: in Moonlight, 2016. Photograph: Alamy

Apparently there are not enough good things you can say about her latest co-star, Jude Law. Harris adores working with him. The two appear in the upcoming Sky drama series, The Third Day, which is being filmed on tiny Osea Island in the estuary of the River Blackwater in Essex. “I’d heard really great things about him in the industry, but he is just such a great human being, with a great work ethic,” she says.

The Third Day, written by Dennis Kelly who co-wrote the Sharon Horgan comedy Pulling, stars Harris and Law as visitors to a secluded island shrouded in mystery and intrigue. Harris is girding herself for swimming scenes due to be filmed in the estuary this month – when the water temperature is likely to hover around 15C. But it won’t be the first time she has suffered for her work. She recalls making 28 Days Later, much of it filmed outdoors. “I was in a little red dress,” she says. “It was winter, I was covered in blood, I was freezing. I don’t know how I survived it.”

The Third Day also means Harris gets to spend more time at home. In London she lives three doors down from her mum, but is working on buying a rural bolt-hole – “off-grid as much as I possibly can be, solar panels, rainwater collection, and growing my own fruit and vegetables, that kind of thing.”

Although she gravitates to warmer climates, she has an abiding love of the English countryside. In an episode of the BBC genealogy show Who Do You Think You Are?, she was thrilled to be able to trace a great ancestor back from Grenada to Somerset, as if it explained some nagging mystery about her love of English villages. “I’ve travelled all over the world and I’m always constantly trying to find a place that feels like home,” she says. “I was, like: ‘There must be somewhere warm that I can move to.’ But no, the English countryside does it for me, that’s it.”

Get me out of here: her breakout role in 28 Days Later, 2002, in which she starred with Cillian Murphy. Photograph: Peter Mountain/20th Century Fox

Unfortunately, the same relative who emigrated to Grenada in the 1820s made his money by trading slaves, a revelation that Harris processes in the show with undisguised torment. When the Windrush scandal erupted, she was brought short by the realisation that her mother, who emigrated from Jamaica at the age of five, could be deemed in some way counterfeit or illegal after spending her life in the UK.

“It could have been her with people banging on the door in the middle of the night and suddenly putting you on a plane, and telling you that a country you haven’t been to since you were five is where you belong,” she says. “It was only because my grandad made sure that all of the family’s papers were in order. My mum remembered the time when he did that, and thinking, ‘Why are we doing this? This is so typical.’ You know, typical of my grandad because he was so exacting and…” She screws up her face in knot of concentration. “What’s the word? Give me a word?” Exacting? Particular? “Like dotting every ‘i’.”

Harris was only eight months out of the Bristol Old Vic Theatre School when 28 Days was cast, but those eight months felt like an eternity. “When I was a child actor, I got everything that I ever went up for, so I’d never heard ‘no’, really,’ she says. “I expected the adult profession to be pretty similar, and then I couldn’t get employed at all.” She remembers trying to figure out her identity in the absence of work. “I thought, ‘If I can’t call myself an actor, who am I, and what am I doing? How am I of service to anybody?’”

So she feels indebted to Boyle, who she says turbo-charged her career not once, but twice – first with 28 Days and then again in 2011 when he cast her as Elizabeth Lavenza, Victor Frankenstein’s girlfriend, in a wildly successful stage adaptation of Frankenstein at the National Theatre. 28 Days scored her a US agent and Frankenstein got her Miss Moneypenny, after Barbara Broccoli and Sam Mendes both came to see her performance. “Generally, when you take big risks, when you do them just for the sake of them, in and of itself, that’s when you get the gifts,” she says.

“When I made Pirates of the Caribbean, people were, like, ‘Now you’re off, now it’s going to be… boom.’ Didn’t happen.” Photograph: Stephen Vaughan/Walt Disney/Kobal/REX/Shutterstock

She’ll soon be gearing up for the promotional tour for No Time to Die, the 25th outing for 007, a movie she has helped to modernise as a thoroughly contemporary Moneypenny. She thinks Bond is a great example of a movie that can be entertaining without forcing viewers to switch off their brain. “In Bond 25 you have four strong, intelligent women who are responsible for driving the plot forward,” she says. “They’ve never had that in Bond before.” As for 007 himself, he’s now in a committed relationship. “He’s not disrespecting women, he’s not after a woman because of how attractive she looks and what quality lay she is. He’s genuinely in a relationship with a woman he respects, loves and cherishes.”

But the gift that keeps on giving is a small 2016 movie that might have flown under the radar in another era. When Harris took a supporting role in Moonlight, a small semi-autobiographical indie with a cast largely composed of unknowns, she had no inkling it would culminate in the 2017 Oscar for best picture, as well as a best supporting actress nomination for Harris’s agonising portrayal of a mother ravaged by crack cocaine. “Three days on a movie made for $1.2m,” she says, still marvelling at the phenomenon of a coming-of-age story with a black gay protagonist at its centre marching away with cinema’s biggest accolade.

“When I made Pirates of the Caribbean, people were, like, ‘Now you’re off, now it’s going to be… you know, boom. Everything’s going to change.’” She pauses, rolls her eyes. “Didn’t happen. And then this low-budget movie that I did for a minimum fee – phenomenal impact.” Phenomenal in what way? “In terms of the roles I get offered. The beating down the door thing I had anticipated earlier in my career and which never happened… well, that’s what’s happening now.”

She thinks films like Moonlight, as well as blockbusters such as Black Panther, are changing the equation of what’s possible in an industry that was hidebound by bean counters more worried about who wouldn’t come than who would. She can remember friends telling her she couldn’t be black and a woman and make it as an actor, but urged on by her mum – “She was always the person who was, like, you can achieve anything” – she persisted. “There are phenomenal black actors who are doing so well in this profession and I don’t think that would have been the case 20 years ago,” she says.

As one of them, she should know. “People are constantly asking: ‘How is it being an actor in your 40s?’ and ‘Isn’t the work drying up?’ and I’m like: ‘No, it’s the best time of my career.’ She thinks Hollywood has finally figured out that they’ve been missing a giant market. “Basically it’s the discovery of the power of the black dollar, right?”

Oxfam fashion raises money to help the world’s poorest people access the basics in life like clean water, and fights for their right to be paid a fair wage and protected against climate change. Find fabulous partywear at Oxfam’s high street shops and online at oxfam.org.uk/shop

Naomie Harris wears clothes from Oxfam’s high street shops and the Oxfam Online Shop. Hair by Ursula Stephen; makeup by Mario Dedivanovic; nails by Momo Qin; stylist Bay Garnett, Oxfam’s Senior Independent Fashion Advisor; assistant stylist Brittany Lovoi; assistant photographer Jesús Baez; shot at Pier Studio