The secret life of Emily Mortimer: The actress talks frankly about her complex world

It's been a challenging time for the actress Emily Mortimer, daughter of 'Rumpole' author John. First she discovered a secret half-brother; then she moved 3,500 miles away from her close-knit clan; now she's starring in a film opposite a sex doll. She tells Demetrios Matheou about her 'rather exciting' new life

This is one of those bracing, uncompromising New York February days, when the icy wind bites between the weave of your clothes; the chill a presage of the snowstorm that will, in fact, hit the city the following day. In such conditions, the actress Emily Mortimer is a real trooper, posing on a balcony of the Chelsea Hotel in an inadequate, if cute, fluffy coat, so that our photographer can capture her in front of the iconic establishment's neon sign. There isn't a hint of complaint or impatience.

James, the photographer, tries to take her mind off the cold with an excited commentary on the hotel, in particular one of the rooms we've rented for the shoot. "It's the fellatio room", he declares boisterously, so named by punters after Leonard Cohen wrote "Chelsea Hotel No 2", in commemoration of the act performed on him by Janis Joplin in the 1970s. I grimace, wondering whether Mortimer really wants to know this. But, having interviewed her before, I should know better; for she is of that certain kind of English, public school-educated middle-class that has a wilfully, wickedly rude edge to its good manners. Two hours later, she will be excitedly telling me about her early appearance in the television show Silent Witness, in which "I had to give a guy a blowjob for a gram of cocaine. My granny watched it – she was still alive then – and she rang up and said, 'Why can't you do a nice little sitcom?'."

Mortimer, then, is as game on screen as she is accommodating off it. Indeed, her best films are often those in which the actress lays herself bare, both emotionally and physically. In 2001's Lovely & Amazing, her character allows her obnoxious partner to analyse her naked body (in a scene for which the actress, unavoidably and bravely, puts herself under scrutiny). In Young Adam two years later, she was entangled in a sadomasochistic relationship with Ewan McGregor, which infamously involved him dousing the naked Mortimer in custard and ketchup, and whose most tender moment involved alfresco sex on a Glasgow dock (she must like the cold). Her latest, Lars and the Real Girl, is a refreshing and daring film, whose co-stars include, most notably, a sex doll; in a non-speaking role, of course, of which more later.

What makes such choices surprising and her performances particularly effective is that, on the surface, she seems so demure, nay English rosy: a perfect complexion matched by immaculate diction and a sweet, quick-to-laugh, eager-to-please demeanour. The contradictions presented by the Oxford-educated daughter of celebrated lawyer and writer John Mortimer are fascinating; yet also, perhaps, a reason why most of her work is in independent films, rather than big-budget mainstream fare.

Now 36, Mortimer has been living in New York for the past 18 months with her husband, the American actor Alessandro Nivola, and their four-year-old son, Sam. She is about to make her off-Broadway debut, in fellow Brit Jez Butterworth's new play, Parlour Song, a three-hander about a suburban marriage in decline, which the actress calls "strange and powerful and something that will give anyone who has ever been in a committed relationship a horrible tingling feeling in their feet".

The production is at the prestigious Atlantic Theatre, founded by David Mamet and the actor William H Macy. If performing here, for the tough New York audience, isn't scary enough, Mortimer tells me that since Oxford she's done very little theatre, anywhere. "Americans like to think of British actors as having spent their lives treading the boards. And as a result they have a sort of misplaced respect for some of us. I've done just two plays professionally – The Merchant of Venice in Edinburgh, and The Lights, an American play, at The Royal Court – and they were years ago."

Moreover, her experience of Shakespeare was humbling. "I thought I was alright in The Lights, and it got good reviews. So when I got this Portia thing, I was so sure I knew what I was doing that I didn't bother to think about it very much. I just got drunk and had fun, and didn't pay much attention to the fact that I had one of the most challenging roles in Shakespeare. Then when I walked on stage on the first night I thought, 'Oh shit'. It was a pretty gruesome experience and scared me off theatre, actually. And I only had myself to blame." Casual indiscretion, lest it not be obvious, is one of Mortimer's most distinctive attributes.

She's glad that Butterworth, director Neil Pepe and co-star Jonathan Cake – all friends – "roped her in" to do Parlour Song. "My apprehension and fear had to be overcome at some point, so why not among friends, in a play that you really think is good, in a great theatre? There is still that terrifying feeling of dead man walking each night, but not of inevitable doom; it might be interesting and some people might like it."

I remark that the preview audience the night before, who contributed some decidedly peculiar interpretations in the post-show discussion, seemed to include very few people under 60. "I think they were 60 when the theatre opened 20 years ago," she quips. "It's fantastic, though. It makes me think how nice life must be when you get to that age, just going to plays and giving your two pennies' worth."

When I last interviewed Mortimer, in 2003, she and Nivola – a Boston-born, Yale-educated Anglophile whom she met while filming Love's Labour's Lost – were newlyweds still debating where to live, it being unthinkable for a young American actor to leave the US (Johnny Depp being the honourable exception), while Mortimer was less than keen to leave England and her family. Although they initially chose Los Angeles, New York has evolved as the practical compromise, with the pair buying a house in Brooklyn.

"I knew that if Sam started to go to school in LA, it would be so much harder to leave," she says. "So I made a campaign to come here, where we could be nearer to England – it's seven hours away instead of 11, so you can go home for a weekend and still feel half-human – and where it was easier to imagine Sam growing up."

In retrospect, she feels fond of LA – or at least Echo Park, where they lived, which has an ethnic mix that includes Hispanics and Cubans, Koreans and Chinese, "and a few hipsters making albums in their living room". When her husband played football every Sunday, "it would be all Mexican – 14 football pitches in a row, people playing music and getting drunk, children running around, grandparents. A real sense of community. That's where I felt most alive.

"But while that ethnic mix is great if you get among it, the city is very segregated: if you live in West Hollywood, you would only see Mexican people when they took your plate from the table. And overall LA just seems so isolated from the rest of the world, which you worry about when you're bringing up a kid. You don't have any sense of what's going on. It's the beauty parlour at the end of the universe, with bizarre, no-season weather, just sun stupidly shining down every day, and people almost in denial of the darker side of everyday life. You know: if you take a pill and go to yoga and drink wheatgrass every morning, you might not die.

"Somehow I find being in New York more comforting – this big city that is dirtier and grimier and where every day you see stuff that reminds you that the world is a difficult, strange place. I wanted Sam to have a feeling that he was part of the world, for better or worse."

They live in brownstone, leafy Boerum Hill, just over the Brooklyn Bridge from Manhattan, where Mortimer is "gradually accepting expatriation in a way that has taken me an awfully long time. I have been really fighting it, as a way of keeping a connection with home." Now, though, having sold her flat in London's Ladbroke Grove, she is ready to add an American passport to her British one, to seal the deal.

She admits to finding it "hard and sometimes heartbreaking" not to be near her family. If this sounds excessive – and she says Americans find it baffling – the Mortimers do appear to be a very close-knit clan. And with her father now 84, she is aware that time together is precious. "I miss him like crazy. I told him he was under strict instructions not to die 'until Sam was old enough to have a conversation with him.And now that's passed, I have to set another rule – I don't know... Sam has to be able to drive him somewhere before he's allowed to pop his clogs."

As well as her father, mother Penelope Glossop and sister Rosie, Mortimer recently acquired a new family member who has added to the reasons for homesickness. In 2004 it was revealed that John Mortimer had a son, Ross, the result of an affair in the 1960s with the actress Wendy Craig, whom he himself was never aware of. Where other families might have been destroyed by the news, by all accounts the Mortimers responded with delight.

"I get on extremely well with Ross. He's just the sweetest companion. And he has a kid the same age as Sam, and they get on really well too. It's been one of the great turn-ups of the past few years, which has done nothing but add a lovely feeling to our family. I think that being as great as he is, and as accepting of the strangeness of the situation as he is, Ross has been an inspiration for all of us."

Bearing in mind this attitude, it seems fitting that Mortimer should be attracted to the script for Lars and the Real Girl, which derives its considerable charm from a situation in which people would be expected to act stupidly and badly, but actually act wisely and well.

The film revolves around the disturbed loner Lars (played by Ryan Gosling), who purchases an anatomically correct, silicon sex doll which he names Bianca, believes to be real and with which he promptly falls in love. "What's subversive about this film," says Mortimer, who plays Lars' determinedly supportive sister-in-law, "is that you expect it's going to be about all the ways in which this person is pilloried and humiliated by the community. And it's more about his coming to terms with himself, and the way the community helps him.

"People are more open to difference than they think they are, especially with people they love, and in small communities," she adds. "I have a very optimistic view about that. You either have to live your life trusting people are more intelligent, kinder, better than they are given credit for, or not."

Not surprisingly, the effect of Bianca on the cast and crew was unusual. "Well, she's a $10,000 sex doll. They are really bizarre, because they are completely realistic. They have everything – teeth, hair, eyes, tongue, mouth, pubic hair... vagina, goodness knows what else. And there's something really disconcerting about being in a room with that person, that thing, because there's no subtext to a sex doll: they're about sad people sticking their willies in holes. It reminds you of the needy side of people. It's threatening, scary."

In a way, however, the theme of the film permeated the attitude on set. "You would have thought the impulse would be to stick cigarettes up her nose and write things on her face and generally humiliate and torture her. But Lars is not some crazy guy banging a sex doll, he's someone who's going through an intense, painful experience. And because of the work he was doing, Ryan was going through all that too. So we tried to be respectful of that."

Mortimer actually suggests having learnt something from Bianca. "I realised from spending lots of time with a sex doll that it is so much cooler to not talk. Because as a result you have a kind of quiet dignity," she says. "Bianca was always sitting in the corner, impassive, not saying anything, and freaking the fuck out of everybody. We felt somehow inferior."

While the garrulous Mortimer has a long way to go before she can exert such silent intimidation, she does seem more self-possessed these days. I ask her how she has changed since being in America. "I think I'm less confused and embarrassed about pursuing a career," she says. "For some reason – neurosis or a sort of middle-class guilt – it was a contentious issue, in my head when I was at home. I was always having to justify it or dismiss it or make out that it was not that big a thing.

"Maybe I would have got bored with doing that at some point, but there is certainly an uncomplicated attitude to work here. That's what is great about America, that people are encouraged to feel good about – it's a horrible phase – bettering themselves. There's nothing wrong with ambition, hard work; you're not made to feel that you have to apologise for it, or treat it as a dirty secret. That is a real relief."

And the career is going rather well. Over a hectic two-year period she has made Woody Allen's Match Point, The Pink Panther opposite Steve Martin, and a slew of independent films – including the train-bound thriller Transsiberian, with Sir Ben Kingsley, and David Mamet's Redbelt, two films in which she is hardening her screen image (her turn in Transsiberian has been described as "a very flavourful performance as a reformed bad girl presented with a whopping opportunity to backslide"). She has also had a quirky, very amusing guest role as Alex Baldwin's girlfriend in the Emmy award-winning television comedy 30 Rock.

She may feel more comfortable talking about her career, but when I ask if she's been recognised by the public lately – she enjoys telling journalists that she rarely is – Mortimer shows that she's lost none of her ability for self-mockery.

"Hey, I was papped – paparazzi'd – in Paris, while we were shooting Pink Panther 2. Sort of by mistake. There are all these famous people in the film – Steve Martin, Jean Reno, Andy Garcia, Alfred Molina. So there were these paparazzi hanging outside the trailers. This one day everyone apart from me had gone home, and I was in my curlers coming out of the trailer to go to a rehearsal. And there he was.

"I was thinking 'Wow, this is amazing, my photograph's being taking by a paparazzi. I must have arrived.' I'm gonna pretend that it's frightfully invasive and everything, but it is rather exciting at the same time. But there was this girl with an umbrella to protect my hair from the rain, and she put it right over my face – sweetly, to protect my privacy. And then the photographer, who was English, shouted out 'Oh yeah, and she's really fucking famous.'"

'Lars and the Real Girl' (12A) is released in the UK on 21 March