Sometimes the Oscar nomination goes to their heads and they start holding court to the world’s media in the five-star anonymity of hotel suites. The hair-and-make-up people prink them up in the morning, the cameras arrive in the afternoon, reality slips a little further from their grasp and they fancy themselves royalty. So how refreshing to be scouring the streets of East London with Sophie Okonedo and catching the café in the Wapping Waitrose just before it shuts. There is a heaving of paper luggage behind us and we are joined by her mother. She is not just her 38-year-old daughter’s bag-carrier in the sense of number-one supporter; she does her hair for her, having worked in a salon, and has got herself lumbered with armfuls of outfits from the shoot. She sets the stuff down, wipes her brow and smiles.

What a picture. There’s Sophie, this startling, stately black woman – you might remember her as the heroic Tatiana in Hotel Rwanda – and there’s her mother, Joan, a white Pilates instructor from a Jewish East End family; Russian on her mother’s side, Polish on her father's. Exotic is the unavoidable word for Okonedo, with her high cheekbones and her tall, regal bearing. Waitrose doesn’t come to a halt at her passing, but it does take stock. The glances she attracts are not the result of fame – she wouldn’t claim to be a household face – but just on account of her looking as she does.

The two may appear thoroughly disparate, but they are a seasoned double act and have caused small public stirs in their time. A good instance was when they were visiting Kenwood House a couple of years ago and a call came through from Okonedo’s publicist Laura to say the actress had received an Oscar nomination for the Hotel Rwanda performance. She told her mother, and the two of them began whooping so crazily that a security guard asked them to keep it down. Joan shared the news with the other visitors, who applauded them as they were escorted out.

When some more recent good news arrived, she was going up Mount Snowdon. It turns out she has become a terrific hill-walker, a passion she shares with her partner, and before you can say Helvellyn, the conversation turns to the late Alfred Wainwright, the crusty old Lancastrian who pioneered some great routes across the landscapes of the North. This time, it was her agent on the phone, telling her she’d been offered a lead in the BBC’s new five-night Oliver Twist this Christmas, with Timothy Spall as Fagin and Tom Hardy as Bill Sikes. Apart from being a good role for her to land, it raises the intriguing prospect of a black Nancy.

“It’s a great script [by Sarah Phelps],” she says. “I read the novel as well, but I thought, better just stick to the script. You can sometimes get side-tracked by the detail of the description in a book. But Nancy is me now, so I don’t have to worry about it. I’ve tended to go by instinct with everything. Sometimes, though, you come across things you don’t know about, like the Rookeries [slums]… London in those days must have been such a frightening place. Wealth in the centre, and grinding poverty surrounding it. The kind of life Nancy had would have been like in a really heavy part of an African city. Life would have been very cheap. She wouldn’t have had a childhood, you can forget about that. Such a rough upbringing – grab, push, take, kill – and yet there’s such natural humanity in her.”

She may not be implying comparisons, yet her own girlhood was not the easiest. She grew up on Chalkhill, a Wembley estate beset by social problems. Her father, Henry, went back to Nigeria when she was five, leaving Joan to cope. Okonedo says matter-of-factly that she has no contact with him. She went to three schools, doesn’t much like talking about them, left at 16 and started work – first at a Covent Garden health spa, then a Portobello Market clothes stall. Two years later, she joined a young writers’ workshop run by Hanif Kureishi at the Royal Court Theatre. Here she discovered she was better at speaking the lines than creating them, landed her first role in Road by Jim Cartwright, and got into Rada. Since then she’s had a fairly constant stream of work, including Cressida for Trevor Nunn at the National Theatre, a fork-lift driver in Paul Abbott’s Clocking Off, and a London prostitute in Stephen Frears’ 2002 movie, Dirty Pretty Things, about an illegal immigrant (Nigerian) trying to make out in London.

Add to this list Hotel Rwanda and then Tsunami: The Aftermath, in which she played a frantic mother in search of the six-year-old daughter torn from her arms by the tidal wave of 2004, and the unavoidable word becomes “harrowing”. She guffaws at this, as if she knows the truth of it but still finds it a strange mismatch for her buoyant self. “You know,” she says, “I was thinking, when I was on the Aftermath set, that I should lighten up, do more comedy. Anything I’m in I wouldn’t really watch myself! Obviously, since Rwanda I have been offered lots of parts set in Africa. I suppose they think, oh, she’s good, and she can play an African. I think that’s how it happens. If I look back over the past few years, it’s true I do attract a certain type of part. Maybe there is something going on inside me that causes that to happen. I don’t know. Perhaps in a different phase of my life I will be doing different things.”

Whatever the reasons, she happens to be very good at the gut-wrenching roles. Worryingly lifelike, you could say. In both Hotel Rwanda and Aftermath, her face, at once resolute and vulnerable, became for long periods the prism through which the viewer’s experience of massacre and natural atrocity arrived. Both were expressive tours de force.

There might have been more work, but for her own childcare arrangements, which she describes as “complex… kick, bollock and scramble. I never know how it’s going to work out, so there’s always a crisis. But I’ve got a great family, with my mother and stepfather living only a mile away.” She has a ten-year-old daughter, Aoife, from a relationship with Eoin Martin, an Irish film editor, and though she is no longer with him, she says he has an active role in Aoife’s life. Even with support from Joan, she’s not in a position to be away working for long periods. Since those two great successes, she has been, “Washing up, tidying up the house [in North London], day-dreaming a lot. You know. I don’t want to be caught in the trap of doing work that I don’t want. I’ve been offered things for quite a lot of money. I quite like low-budget films. I can sniff things that are bogus and I don’t want to be involved with them. I don’t mind if it’s a comedy or a musical, or whatever it is, but there has to be some central truth about it or else I’m not interested. Same with the people I work with. I can’t stand bullshit.”

Some of this robustness comes from her indestructible grandparents of 96 and 94. Even knowing their origins, it still comes as a surprise to hear her talk of them. “Still together, still arguing like mad, still speaking in Yiddish. He was in the rag trade, in schmutter. She decided to be an artist in her late middle age, and went to study at the Slade. They have photos from the turn of the century, and I always remember getting the boxes down from the shelves in his room, and him saying, ‘Now that was in 1905.’ We got them out yesterday and we were all looking at them, me and my mother and my Uncle John. I don’t often talk about my background, and when I do, people get a bit shocked.” She was always, she remembers, the only black girl in the synagogue when she went there with her grandmother.

And now, she’s fairly sure, the only black Nancy. It could be an inspired piece of casting by director Coky Giedroyc, for she is perhaps the least categorisable and most border-crossing of all the characters in Oliver Twist. She seems to be the only one who understands both good and evil and is not exclusively confined to either realm. But she is a morally ambiguous soul, with a tragically unconditional love for Bill Sikes that brings her down into crime and self-destruction. It will be harrowing, all right. But don’t be deterred – it’s bound to be comic as well along the way.

Oliver Twist begins on December 18 on BBC One

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