Sam Mendes is no slouch when it comes to casting. The director, and latter-day partner to Kate Winslet, has discovered a few American beauties in his time -- Thora Birch and Mena Suvari for starters.

But for his latest film, Away We Go, it's the turn of a British talent -- albeit one who's much better known over the pond -- to be in the spotlight. Still, unlike American Beauty stars Birch and Suvari, Carmen Ejogo is 33, married with children, and hardly fresh out of drama school.

When we meet, it's the day after the UK premiere of Away We Go. Wearing a black dress and platforms, she looks nothing short of dazzling. With a mass of frizzy black hair, smooth caramel-coloured skin and penetrating brown eyes, her elegant demeanour gives her the air of a more approachable Thandie Newton.

Her accent is by way of her west-London upbringing, though, as she tells me: "I prefer not playing English. The occasions that I have, recently at least, it's felt a little too close to me, and it's not as much of a transformation."

This telling comment aside, the fact you've probably never heard of Ejogo is surprising. Firstly, with a surname as distinct as that -- her father was Nigerian -- you'd think it would've lodged in the collective consciousness by now. Secondly, given her beguiling beauty -- she was a child model -- you'd think that face would be on every billboard. And, thirdly, she started acting in 1986, when she was a teenager, in Julian Temple's musical Absolute Beginners. Quite simply: where has she been all our lives?

The short answer, at least for the past decade, is in the States. "I really did turn my back on the UK," she says. "I thought, 'America's where I'm going to do my thing.' Then I got there and America was not what I thought it was going to be at all. To couple making good money with good work is a much harder task than expected. The kinds of roles on offer are just not that interesting. I just don't find them appealing very often."

If Ejogo sounds a little bitter, it stems back to 1997, after she appeared in Metro, a forgettable action film with Eddie Murphy. When she "started traipsing around" the endless meetings required of fresh-faced actors in Hollywood, she received a shock to the system: her skin colour was a barrier.

"I was told, 'The head of such-and-such agency doesn't think there's enough work for someone who looks like you do. What are they going to do with you?'" Is it still this bad, I ask? She shakes her head. "That's not the situation any more. I think it's changed a lot. It's fantastic that it's moving in a positive direction. But I've become disillusioned in the process."

At least, her time in the States introduced her to her husband, actor Jeffrey Wright. Some nine years her senior, Wright is best-known for playing CIA agent Felix Leiter in the Daniel Craig 007 films. But, as I tell Ejogo, his performances in films like W (he was Colin Powell) and Basquiat (he played the titular artist) prove he's one of the best actors working in America right now.

"He's one of the best actors, period!" she shoots back, grinning. Ejogo met Wright on Boycott, a TV civil-rights drama in which he played Martin Luther King to her Coretta Scott King. Hooking up in Atlanta, they took off for a while -- "we travelled around a bit, bumming about, as we're both quite nomadic" -- before marrying nine years ago. After they settled down in Brooklyn, Ejogo gave birth to two children -- seven-year-old Elijah and three year-old Juno. It's evident she and Wright are a good match.

"We definitely see eye to eye in terms of the type of work we choose. He has an easier time of it. He didn't take time out to be pregnant with two kids! Being a woman in the business limits your choices."

Before she met Wright, she scuttled back to Britain after Metro and took a role in Michael Winterbottom's dark sexual drama I Want You before finding herself in another risible blockbuster, The Avengers, alongside Uma Thurman and Ralph Fiennes. Yet, since then, Ejogo has avoided the crass commercialism of Hollywood.

"I've been there," she sighs. "I've been there enough times to now be happy to work once a year with someone that's really worth working with, and otherwise I'll just hang out with the kids. It really is too heartbreaking as an artist... I'm not being over the top, I really mean that."

She certainly doesn't feel that way about Away We Go. Written by husband-and-wife authors Dave Eggers and Vendela Vida, the plot follows thirtysomethings Burt (John Krasinski) and Verona (Maya Rudolph) who are expecting their first child when they begin to get the jitters about where they belong in the world. Verona's parents have died in an accident years earlier, and Burt's decide to head off to Antwerp. And so the Colorado couple take a road trip to help them decide where to relocate.

"I relate to that hugely," admits Ejogo. "Particularly that sense of, 'what is going to constitute the kind of family that I'm aspiring to create for myself?' When I was first pregnant, I had just moved to New York with Jeffrey. It was in 2001 -- and my baby was due in October. September 11th happened, and I was eight months pregnant."

A picaresque comedy, Away We Go jumps from city to city as Burt and Verona visit friends and relatives. Ejogo plays Verona's sister, Grace, now a hotel manager in Arizona. The scenes she has are among the more touching in a film that has a tendency to veer wildly towards comic caricature. Grace has dealt far better than Verona with her parents' death.

Ejogo's own "wheeler-dealer" father, Charles, died in a car crash when she was young. It meant she understood "quite intimately" that dynamic. "I'm probably the sibling, in my family, who was able to be more open about it." Ejogo's mother, Liz, who hails from Cumnock in Scotland, was just 17 when she met her father. Their relationship only lasted two years. Growing up while her mother took cleaning jobs to make ends meet, they lived in affordable housing near Chelsea.

"Some of the most dysfunctional families you'll ever come across, I grew up around," she recalls. "But for me that's just normal. As I strive to define my own family, I still can't figure out what I think the perfect family is." I wonder if it was her mother -- "a total nomadic hippie spirit", as she calls her, who is now "out there living life" -- who inspired her towards her calling.

"To some degree, yeah," she nods. "Watching an amateur drama queen infuses one with certain acting traits." She's smart enough, however, to look deeper than this. "About 80% of actors come from broken homes, so there's some kind of trauma at play. You're observing so many experiences from such a young age, you have this deep well to draw from when you take it into your career." It's certainly what Mendes spotted. Now Ejogo just has to hope others do too. HQ

Away We Go opens on September 18th

- James Mottram