Emma Thompson: 'Family is about connection'

An unremarkable row of Victorian houses, a quiet suburban street, parked Fords and Vauxhalls. Nothing remotely fancy, which is why it seems incongruous to be suddenly face to face with not just one but two famous actors: Emma Thompson, and her mother, Phyllida Law. The person I'm here to see is Thompson; but I've pressed the wrong doorbell and disturbed Law, who lives next door. Thompson, though, has heard the noise, guessed at my mistake, and seconds later is opening her front door too. Which is how I come to find myself, on a mundane grey morning in north London, sharing a giggle with two film stars over the bathtub in Law's front garden, complete with a pair of mannequin's legs sticking out. "It is extraordinary," agrees Thompson, as she ushers me through the correct front door. "That was the tub in our house when I was growing up – I had all my adolescent baths in it."

The vignette seems to sum Thompson up: ordinariness laced with the right amount of glamour (without makeup in the harsh daylight and at 50, she's looking great), a good helping of wackiness and oodles of family references and family history. In fact, family is her metier. As well as having her mother as a neighbour, her younger sister Sophie (another actor) lives a mile or so away with her husband and children. Thompson's daughter Gaia, 10, skips to and from her granny's house, much as Thompson herself did in this very street as a child: she grew up just down the road and her grandparents also lived on the same street.

"Family is the centre of everything for me," agrees Thompson. "But family is about connection, not necessarily about blood ties. It's about extended family – and extending family."

What's important to her, she explains, is a wider, welcoming, nurturing group of people who have put down roots together – and here in her west Hampstead nest, she's surrounded by her family and its extensions. As well as Gaia, her husband Greg Wise (yet another actor), her mother and sister, there's Tindy (Tindyebwa), a 23-year-old former child soldier from Rwanda who Thompson and her actor husband Greg Wise welcomed into their family via an informal adoption seven years ago: he is now a postgraduate student in London and has his own flat, but is often around at weekends. Then there's her PA, Viv, in the office: she used to be Gaia's nanny and clearly feels part of the clan too. It's this idea of the welcoming family, accessible to outsiders, that lies at the heart of the new Nanny McPhee film too – as with the first, Thompson has written the screenplay and plays the leading role.

The Nanny films, which she adapted from Christianna Brand's Nurse Matilda books, are the passion project of Thompson's life – she has spent a massive chunk of time on it. "It goes back 15 years – I started work on Nanny McPhee long before I had Gaia, so everyone who thought I was writing a children's film because I'd had a baby was wrong."

As a character, Nanny McPhee is devoted to caring and enabling: and this, I reckon, is the role Thompson most wants to play in real life. In the film, she dispenses goodness, wisdom and life-enhancing support: in reality, she is as aware as anyone that her influence springs from being a famous film star. Though she's happy to use this, as she strides into battle against everything from human trafficking to the proposed third Heathrow runway, what she hopes is that people will see her for what she is, rather than who she is.

Motherhood, says Thompson, has grounded her. Gaia is in many ways her miracle child; conceived via IVF, born as she was nearly 40. She gave birth without drugs, and says she can't imagine why any woman would ever elect to have a caesarean. "Even now, when things are bad I go back and I remember the birth process. I can transport myself back to that moment when Gaia was born – it's like a well from which I draw strength."

After Gaia there were three more gruelling years of IVF. Its eventual failure was a particularly poignant blow to Thompson: her father and a much-loved uncle had died young, casting a shadow across an otherwise idyllic childhood, and she had always vowed to "bolster the numbers" for the next generation. "I grew up in an incredibly happy family, but it was damaged by this physical trauma."

Giving up on future pregnancies was, she admits, tough: but looking back now, Thompson can see it in a positive light. "I couldn't have more children, and that was hard; but perhaps if I had [had more], I'd have missed out on this extra act of mothering that I've had with Tindy. Because there was space in my life for him, and I don't think there would have been space if I'd had another young child around."

Thompson met Tindy when he was 16 at a party organised by the Refugee Council. His father had died of Aids when he was nine, and his mother and sister had disappeared during the Rwandan genocide. Tindy himself had been press-ganged as a boy soldier, before fleeing the country with the help of an aid agency. Once in Britain he was homeless and friendless, and at one point was forced to sleep rough in Trafalgar Square. But the Refugee Council helped him, and Thompson and Wise were so struck by his story that they invited him to their home for Christmas. After a while, he became an unofficial member of the family.

In the early days, Tindy was prone to nightmares, and he has written about his "on and off depression". But it's clear that Thompson has played a crucial role in helping him to turn his life around, and even though he's now an adult, Thompson hints that at some point in the future, she and Wise might become his official parents. "The important thing, though, is that he is in our life and we are his family, and that's an absolute truth."

From Tindy springs inspiration for her work campaigning for the world's disadvantaged (she is an ambassador for Action Aid) because it has enabled her to make a far bigger commitment than money. Adopting disadvantaged babies from other parts of the planet, Madonna and Angelina-style, grabs the headlines but it takes real guts to welcome a traumatised teenager from another culture into the heart of your family. And having Tindy in her family makes a difference to her work on behalf of people who do need help. "It makes the things I talk about real – I talk about them, and then I come home and there's this other dimension to what I've been talking about. It's all connected: Tindy and I may work together, one day."

On a mother-son level, their relationship is, she says, complete bliss. "He's serious-minded but very funny too. We laugh a lot. And we talk a lot – we go for long walks to chat, which is lovely." He and Gaia are very close – Tindy has been to Gaia's school to teach her class Rwandan songs, and they love spending time together. "Gaia was nearly three when he arrived," she says. "He became her big brother very quickly – she adores him."

Though their backgrounds could hardly be more different, Thompson says she and Tindy are on the same wavelength. "We're so alike … it's as though we were related." This is evidence, she says, of the unexpected joy that can come from exploring an unlikely relationship; she feels that human connections can spring from bonds that are far from obvious, and that we all enrich ourselves by being open to exploring those connections.

Another well in her life is her relationship with Wise, whom she married seven years ago when Gaia was four. He is, she says, hands-on as a parent. "He can do everything I can do. He loves home life – he loves carpentry, he loves making things. But he's not entirely domestic – he loves acting too. I'm the main breadwinner, but in a marriage like ours there has to be constant role negotiation. It can't just be that the person who earns the money does what they want to do. You have to learn respect, and you must have that for the other person's needs and desires."

She's not starry-eyed about marriage, saying it's "like a vessel you have to sail in for a long time. You can't afford not to do the upkeep required, and sometimes you have to give it some attention – as in, we've sprung a leak, what are we going to do to solve the problem?"

She knows what she's talking about: her much-publicised partnership with the actor and director Kenneth Branagh lasted five years before ending in divorce in 1994. This time, though, Thompson gives every impression of being on a long-term voyage.

Wise, she says, did the bulk of the childcare during filming of Nanny McPhee, though Gaia did sometimes join her on set. But this summer, she plans to be at home. "I always knew I'd want to be the sort of mother who would be properly around. What I most wanted was for Gaia to take me for granted – I couldn't bear the thought of being that 'special' kind of mum, I wanted to be an everyday kind."

There are obvious parallels between her daughter's childhood and her own: her father was the originator of the much-loved children's TV programme, the Magic Roundabout. "It was a hidden kind of fame," she says. "We really enjoyed it, we watched every episode. With Gaia it's different, because she doesn't really like watching me in other movies – though she doesn't mind Nanny McPhee, because I'm in disguise."

Today, Wise is away in York. "So," says Thompson, "I'll collect Gaia from school and I'll be with her this evening." Then she realises something and her eyes shine at the idea of it. "In fact, Greg will be away overnight so Gaia and I will be able to have a girly evening together. I think we'll watch a movie and then she can sleep in my bed." The idea sounds so appealing that she claps her hands together.

Nanny McPhee and the Big Bang opens in cinemas on Friday 26 March