Interview: Emily Watson, actress

It's a tiny moment, a snippet of conversation in a local pub between a middle-aged barmaid and a stranger, played by Watson. For a fraction of a second, before the full introductions have been made, the barmaid thinks that Watson's character is the baby she gave up for adoption more than 30 years before, and in that second, decades of pain and loss and now, heartbreaking hope, flash across her face.

"You just talk about it and…" Watson gasps and puts her hand to her chest, "that's the heart of the film: that terrible separation of the child from its mother and neither of them knowing what's happened to the other. Devastating. It slams into your heart."

The film is based on the story of Margaret Humphreys, a social worker (Watson) who by chance uncovered a social scandal which ultimately led to official apologies being made by two governments on opposite sides of the world.

It was 1986. Humphreys was working in Nottingham when she was approached by an Australian woman who had travelled to the UK to look for her birth mother. After agreeing to help with her quest, Humphreys unearthed a British government policy that had resulted in around 150,000 children in care being forcibly deported to Commonwealth countries, mainly Australia. Many, some as young as four years old, were told their parents had died and were then sent overseas to church-run or other institutions, where some were used as forced labour and many were abused. The promise was "oranges and sunshine", the reality was something very different.

Based on Humphreys' book, Empty Cradles, the film tells of how she became the conduit for thousands of people searching for their birth mothers, trying to understand what had happened to them and in some way salvage something from their blighted childhoods.

The fact that these events happened so recently (the British government only apologised to the child migrants in February last year) only adds to the shock. Contributing to the intensity is the fact that the story is told with no fuss, no histrionics, no great outpourings, in a sense, nothing really cinematic. The choices and the slow pace of the film limits its power as entertainment, but it does nothing to lessen the story. Or the central performance.

Watson has a reputation for being utterly fearless as an actor. How else could you describe her role as the tortured Bess in Lars von Trier's Breaking the Waves, the breakout part for which she was nominated for an Oscar? Or her interpretation of cellist Jacqueline du Pre in Hilary and Jackie, for which she was nominated a second time. Or even her less harrowing but no less complex roles in Synecdoche, New York, in which she stands, naked, in a dingy hotel room ruminating on loneliness, or in Gosford Park where her performance as the hard-headed maid Elsie, the beating heart of the film, was a high point, elevating her above a truly stellar cast.

She has chosen roles that demand emotional complexity, a willingness to delve into the darker areas of a character's psyche. But there's something else about her too, a quality that is increasingly rare amongst cinema actors and is therefore all the more precious. Emily Watson is inconspicuous.

Of course, anyone who has a critically acclaimed film career of nearly 20 years duration is hardly unnoticeable, but it's in the way she is, the way she looks and handles herself, at least in the public eye. Low key might best describe it. Or perhaps ordinary, although that sounds pejorative, or unstarry. Either way, it's why she is perfect for Oranges and Sunshine.

The film is the feature debut of director Jim Loach (son of director Ken Loach) and is written by Scottish writer Rona Munro. It took Loach and Munro years to convince Humphreys to sell them the rights to her book, partly because she was apprehensive about how her story would be translated to the screen. But also, one suspects, because it's just not really Margaret Humphreys' style.

Loach put together an exceptional cast including Hugo Weaving (Lord of the Rings) and David Wenham (Australia) but felt the portrayal of Humphreys would be key to the film. In Watson the director found someone convincing as a woman who undertakes heroic feats quietly and diligently, with very little fuss even when it ends up costing her a great deal. She's a heroine who hardly speaks, who spends the film simply listening to the stories of trauma and abandonment and tries her best to do what she can to help. It takes an actor of Watson's ability to bring that complexity to life.

"I'd never heard of Margaret Humphreys before I was approached about this film," she says. "She's not the sort of person to put herself forward, I guess, but she single-handedly did everything. She's an amazing, incredible force of nature, I think.

"She was quite reluctant to do what she did, in a way. It just sort of happened to her. It was that social worker instinct of helping people and not saying no. And then, of course, it took over her life."

Watson has played real people previously, but as to how she approaches that, she says it's never the same twice. In this instance, she did meet Humphreys, but only after filming was complete.

"It's difficult," she says. "I do it differently every time. But I weighed it up and thought, you know, I'm not going to. I'd seen her on video and I thought the way that she is in person might, this sounds weird, but I thought she might be a bit distracting."

Humphreys' two grown-up children did come to the set when the crew was filming in Nottingham.

"I could see they were wary," she says. "You know, this is our life you're filming. But I could also see they are a very strong family."

Watson did watch a documentary in which Humphreys sat with one of the child migrants, now adult, and explained that she had located his birth mother. It was, Watson says, her way into how to play Humphreys on screen.

"She has the most unbelievable restraint," she says. "She only ever gives the information that the person is ready to hear. For me that encapsulated what she did. She was only ever following up what people wanted her to do and what they wanted was utterly life-changing, desperate stuff, but there's something of the social worker training in there which is about listening and waiting, listening and waiting."

Indeed, Oranges and Sunshine is no searing drama. There's no great Hollywood-style moment of vindication - this story is based on reality, it's not a fiction with the neatness of a resolution where it all comes right.

"There's no moment in court, nothing like that," says Watson. "Margaret says at the end that everyone thinks there's going to be this amazing moment of restitution but of course there isn't.

"But that's the reality of it. It's not Erin Brockovich. There's a multitude of lives thrown away out there, and relationships destroyed, that will never be recovered."

Watson pauses and for a moment looks a little drawn. Partly it's the subject matter of the film, of course, and as the mother of a two-year-old and therefore "my brain is shot". But there's more to it than that.

To describe Watson as a reticent interviewee wouldn't be quite right. She's polite, friendly even, in a low-key sort of way. But it's clear, even if it's unspoken, that she's not about to offer everything up, to pour her life out for other people's consumption. She wants to talk about the work. Actually, maybe she doesn't even really want to talk about that, not to journalists anyway, but she knows she must and so she does. As for the other stuff, it's a no-go area.

Part of what has always intrigued people about Watson's ability to play damaged characters is that her own background seems so straightforward. She grew up in Islington, the daughter of a teacher and an architect. She studied English at Bristol University and then spent a year at the Drama Studio in London before she got a contract at the RSC. It was there that she met Jack Waters, who was then an actor and now writes screenplays. They married in 1995 and have two children, Juliet, five, and Dylan, the two-year-old.

Watson thinks that Humphreys must be an extraordinary person because as the mother of two young children, if people on the other side of the world were writing to her asking for help, she wouldn't do it.

"I wouldn't ignore it," she says, "I might try to find someone else who could deal with it, but I wouldn't take it on."

Her phone rings and she apologises. "It's all going off a bit in my domestic life," she says, "so I just need to make sure that's not the nanny."

As interruptions go, it couldn't really be more apt as we'd been talking about how Humphreys became a mother figure to the many survivors who contacted her. Watson looks at the screen and it turns out to be the estate agent. She hangs up and zips the phone away in her bag. It's stresses all round, it seems.

"You have, no idea," she says. "No idea." But she doesn't offer any more, or look like she'd welcome any prying, so we talk instead about her acting.

The roles that Watson has become best known for, those "crying and dying" ones, have, she says, sometimes been hard to keep under control, to stop from bleeding into her own life, more so once she had children. "As I've got older I've got much better at knowing, yes, this is going to be an incredibly intense day but once it's done I'm going home, hopefully in time to put the kids to bed but if not, we'll have dinner and I'll relax. Normal life will resume and then I'll do it again tomorrow."

It is, she says, an "immense effort to keep the whole circus on the road" but it works and she works. But things didn't quite go to plan with Oranges and Sunshine.

"I shot this film in six weeks," she says. "It was one of the toughest six weeks of my life because real life goes on," she pauses. "My mother died in the middle of the shoot so it was really, really, really tough. I've got better at compartmentalising but I've got to say, I was not prepared for the way it panned out. It got pretty dark. It was all quite a rough ride."

Watson's reticence suddenly becomes a lot more understandable. It's difficult to imagine what it must have been like playing a woman whose life's work is reuniting children with their mothers, when in her own life, she would never see her own mother again.

"Once we got out to Australia to shoot I was just in survival mode," she says. "I just had to think about getting through the day."

Again, Watson looks distracted, as though her mind is on other things. But we talk again about the film and Watson says she's delighted that in special screenings organised for migrants in the UK and abroad, the reaction has been positive.

"They've loved it," she says. "And the social workers too. They're loving it because they get such a bad rep in films and on television and this shows just how hard their job is and the judgments they have to make." She pauses. "And the sh*t they take."

Maybe that's part of why Watson was so drawn to playing the role of Humphreys. It was a chance to show someone being ordinary and extraordinarily and demanding no attention for it. Just getting on with it.

"She's an extraordinarily moral, good person," she says. "And you don't often get to play that in films. It's very inspiring."

Oranges and Sunshine is on general release from Friday

• This article was first published in The Scotsman on March 26, 2011