The first time I met Naomie Harris was in 2010 in a noisy cafe in Portobello Road, London. I thought then that she was the sort of girl you would have wanted to make your best friend if you had met her at school: warm, talkative, not at all puffed up and not dressed up either. I don't remember what she wore but it wasn't aiming to be memorable. The cafe was noisy so she suggested we talk in her nearby flat. She was best known then for her role in Pirates of the Caribbean and for television dramas (Clara in White Teeth, Hortense in Small Island). She was about to pull into the fast lane, but had not yet accelerated into being a Bond girl (or as she prefers "Bond woman") in Skyfall. Nor would she have had any inkling that she would take on Winnie Mandela in a £22m biopic Mandela: Long Walk to Freedom opposite Idris Elba as Mandela (opening in the UK in January).

Today at the Dorchester, the quick-witted readiness to talk, the sparkle and infectious peals of laughter are as I remember. But the look is transformed. The 37-year-old is dazzling: 100% film star. I fail to ask who designed her close-fitting black and white dress because I am focused on her insanely high Louboutins, their scarlet soles flashing as she walks in. She protests: "I am always tripping over, very much a klutz." And being Naomie Harris, she is relaxed about her transformation.

"As a child, I used to wear my cousin's hand-me-downs and be perfectly happy. I was like: why is anyone interested in fashion?" Being in "the business" changed that: "I realised the power of clothes to change the way you feel. If you wear dowdy jumpers, you feel dowdy.

"If you dress differently, you feel differently," she continues. "You know what? It is a celebration of you. As you come more into yourself, you want to celebrate yourself more, and that is a beautiful thing and an important thing – as you get older, you should fall more and more in love with yourself."

I prefer her thoughts on jumpers. On the other hand, celebration has been the order of the day for Harris, not least because Mandela: Long Walk to Freedom, based on Nelson Mandela's autobiography, has been getting the red-carpet treatment worldwide. The Johannesburg premiere was especially daunting, Harris tells me. Winnie Madikizela-Mandela – now 77 – was present, as was Mandela's third wife, Graça Machel. Ahmed Kathrada, in prison with Mandela on Robben Island, and lawyer George Bizos, who defended Mandela half a century ago, were also in the audience.

"It was nerve-wracking," she says. "When we showed the film in Toronto, people were laughing, responding noisily. In South Africa, there was silence. I thought: oh no! But afterwards, people were in tears, incredibly moved, processing the film on a profound level. One of Mandela's nieces was sobbing in my arms."

The Mandelas are – naturally – the film's ultimate critics. At 95 and after three months in hospital with a lung infection, Nelson Mandela, who lives in the suburbs not far from where the film was being shown, has not seen it. But he has watched a clip in which he is said to have mistaken Elba for himself. Winnie has been outspoken in her approval and got treated to ululations at the premiere. She dubbed Harris and Elba "honorary South Africans" and said it was "the first time [she has been] truly captured on screen".

Harris is over the moon – "It is the greatest accolade possible" – and is, unsurprisingly, careful not to say anything critical. You see her difficulty: Winnie Mandela will never be a less than controversial figure but Naomie Harris's task has been to treat the transition from innocent young woman to violent demagogue sympathetically – or at least with understanding.

She admits that playing Winnie Mandela is "the hardest thing I've ever done". Learning to speak with a Xhosa accent in front of South African actors required nerve. And she had never played someone who ages: "I had to plot it out. At 21, Winnie had joyfulness, innocence, openness, excitement and an ability to fall in love deeply. But as people get older – well, some people – they close down. Winnie is a complex woman – like seven people in one. She was a grass-roots activist – on the streets with the people. She could only survive the level of brutality she went through this way." Harris has extraordinary ferocity in the role: she is at her most moving when most militant, punching the air with her first, crying: "Amandla!"

The film's second high-profile screening was in Washington – at the White House. Harris describes Barack Obama coming into the library to meet the cast: "He lights up a room, he is so magnetic and charming. He said: 'The last time I saw you, you were kicking butt in Skyfall.' And I thought: 'Oh my God, the President knows who I am!'" But we will have to wait for Obama's verdict – he introduced the screening but was too busy to watch the film. (Michelle Obama has promised Harris that she and her husband will be seeing it together soon).

It is Harris's ability to convey happiness on screen (as when she falls in love with Nelson) that is her most striking quality and defines her other work too. (She had a memorable radiance and a deliciously teasing manner as a hard-working Kenyan schoolteacher in The First Grader, directed, like Mandela, by Justin Chadwick.) What makes her happiest in life? "Simple things. I love being in the countryside. I adore family. The more artifice involved, the more uncomfortable I become." When her family visits her on set, they tell her she seems at her happiest in front of a camera. "I feel comfortable because I've been doing it since I was nine [in children's television dramas]. I love being someone else, accessing different worlds – there is joy in that." Her dream of future happiness is not elaborate either: "When I retire from acting, I'd like to have a home in the countryside and write. I wrote a novel when I was 13. I imagine a south-west facing house and rolling hills. It is sunny, so probably not in England… though it is always summer in my imagination…"

What was it like playing opposite Idris Elba (star of The Wire and Luther, and recently voted Harper's Bazaar's man of the year)? "He's playful, mischievous and has a very silly sense of humour. I'd always make him laugh for some reason. And he has this little giggle and it's really cute. But the wonderful thing was that, after the first rehearsal, he came up to me and said: 'I am scared. Are you scared?' He made me feel so much better by being open about it." She adds that they bonded over a shared birthday, 6 September, and because "he is an only child and so am I".

I remember last time we met how present her mother, Lisselle Kayla, was in Harris's conversation – as an empowering figure. Her mother raised her on her own in Finsbury Park, north London (her Trinidadian father, Winston, left when she was little) and their relationship seems always to have been tremendously close. Yet she is undeceived about single parenthood.

"A child needs male and female influences. It is important for their wellbeing and sense of being a whole person. As a child, I thought my mum was a kind of god who knew everything and was always right, and actually life is never like that. It's a compromise, a balance…"

I ask at what point it occurred to her that her mother was as faulty as everyone else. "I am still realising it! I still think she is so amazing." Her mother used to be a scriptwriter on EastEnders and is now a healer who helps Harris overcome performance nerves. "She does EFT, emotional freedom technique, about releasing childhood traumas impacting on your life through a process of tapping along your meridian points."

Thanks to her mother, she was able to trace her nerves about having to give a speech back to being 17 and at school. "I had to give a speech in a sociology class and it went really badly. I had done so much research, I got overwhelmed and froze. I felt really awful… But we tapped through that and it has made me much more confident."

Naomie loved Finsbury Park and still does because it is "so multicultural. I had no idea growing up that I was in an ethnic minority. There were so many black, Asian and oriental people – a real mixture." And she still remembers her first stabs at acting: "My grandad gave me a children's Bible and I was obsessed with it. Whenever anyone came through the door, I wanted to perform the Adam and Eve story."

She grew up without knowing her father and then, encouraged by a friend, met him when she was an adult. She says: "Unfortunately, I can't talk about that now. My dad asked me not to, as he does not have any recourse to tell his side of the story, and I respect that." And if there is a man of a more romantic sort in her life, she is not in any rush to talk about him, either.

The director Sam Mendes once described Naomie Harris as "gentle and loving" but also as a "ditherer". Does she recognise herself in that description? "Um. I don't think I really… I don't know because I dither sort of… I think of dithering as someone who is unsure constantly and I am very certain but er…" I laugh and interrupt her to tell her it is a dithery answer. But evidently once she has set herself a task, she sticks to it. Last time we met, she told me she was bullied at school. Did this help her develop a thick skin? She replies that it was independent-mindedness that got her bullied in the first place.

How much does she care what other people think of her? "Not very much at all. I care what I think about myself. I am always trying to find what the truth is for me and, weirdly, that is one of the things I find hardest to do." Her determination must have helped her get into Cambridge university, where she read social and political sciences and where the novelist Zadie Smith was a contemporary. ("She was the cool one. I was the nerdy one with glasses.") Although she was not happy there, she insists: "Going to Cambridge is one of the things I am most proud of. I learnt so much."

Finally, I ask about South Africa. She hesitates. You can see it is a question to which she has given much thought: "The legacy of apartheid is incredibly apparent still. It is extraordinary that a structure like apartheid was dismantled and a new regime put in place without [a war] but, in a way, this hasn't allowed people the opportunity to vent their grievances, to get a sense of justice. It is a beautiful country and the people are open, generous and warm. But South Africa still has a lot of healing to do."

Our conversation is almost over when the door opens and Idris Elba walks in, saying to Naomie that he needs a word. They disappear briefly and, on the other side of the wall, I hear laughter – they sound like conspirators. On their return, he apologises and exclaims in a way that manages to avoid out-and-out thespiness and sound heartfelt: "It was a moment I needed with my co-star, who I love. We really helped each other get through this film."

Mandela: Long Walk to Freedom opens on 3 January