Emily Mortimer - A Sloane from the Chilterns who loves life in the fast lane

Emily Mortimer thinks her role as the voice of a sexy sports car is the most glamorous she's had. Gill Pringle catches up with her

The daughter of celebrated actor and lawyer Sir John Mortimer of Rumpole of the Bailey fame, Emily Mortimer enjoyed the quintessential upper-crust upbringing. Raised between Chelsea and the Chilterns, she attended St Paul's Girls School before reading Russian at Oxford where she performed on stage. Today, she leads a remarkably different life, fully immersed in her adopted US homeland and living with her American husband and their two children in Brooklyn.

Originally, she hadn't planned to follow in her late father's footsteps, instead adapting a screenplay and writing a newspaper column. "Acting was always something I pretended I didn't want to do when I was growing up," she recalls. She didn't even think she had the sort of looks that translated into screen stardom, explaining today how she was thrilled to voice a sassy sports car in animated movie Cars 2, because audiences wouldn't be able to see her "depressing" hair.

"I'm convinced that this is the most glamorous I'll ever be on celluloid in this film. And luckily I don't have any hair because my hair has always my downfall. My hair looks really bad in all my movies so I was delighted to be a car that doesn't have hair. I look better in this movie than I ever have or ever will look in any other movie," she laughs only half in jest. "Even my agent complains about my hair."

Delightfully ditzy and utterly charming, she still struggles with insecurity regarding her dramatic abilities – even when in a sound booth voicing a car. Most actors would spin some story about how they had a particular Bond girl or sexy spy in mind when it came to voicing a curvaceous sports car/spy named Holley Shiftwell.

Instead, she says, "I couldn't do that because I knew if I tried to emulate a Bond heroine I would just fall flat on my face. I just had to be me in that world. And the script was so complex. What you are given is very hard to navigate because it's mainly for the animators really and so you're completely relying on the director to paint the picture. It's a very organic process, certainly with my character anyway, nobody really knew completely where she was going to go. My natural instinct was just to do what I would feel, which is just completely dippy and pathetic in that situation and that's what I was doing," she says shyly. "Then we realised that if she was out in the field in this very high-powered spy mission, she wouldn't be quite so crap as that so we developed it together over time."

Despite her timidity, she has validated her career with a series of fine performances. Making her TV debut in the 1994 comedy Under the Hammer, written by her father, she then popped up in a single episode of Aussie TV series Blue Heelers before winning her first major role in a TV adaptation of Catherine Cookson's The Glass Virgin in 1995.

A year later, Hollywood came calling, appearing in her first major film, The Ghost and the Darkness, opposite heavyweights Michael Douglas and Val Kilmer, prompting her to say at the time. "This is not meant to have happened to me at all. I am a Sloane, from the Chilterns."

After numerous high-profile films, including Notting Hill with Hugh Grant, The Kid with Bruce Willis and Woody Allen's Match Point with Scarlett Johansson, she's become accustomed to her status as one of the most in-demand British actresses in the US although she still insists that directors call Kate Winslet or Kate Beckinsale before her.

Like many marriages made-in-Hollywood, Mortimer met her future husband, actor Alessandro Nivola, on the set. Both cast in Kenneth Branagh's Love's Labour's Lost in 2000, Nivola was King Ferdinand to her Katherine. Three years later they wed at Mortimer's family home in Buckinghamshire, today proud parents to son Sam, seven, and 18-month-old daughter May.

Motherhood has barely slowed her down. Recently appearing with Leonardo DiCaprio in Shutter Island and opposite Michael Caine in Harry Brown, she completed filming Our Idiot Brother with Steve Coogan earlier this year and is currently shooting thriller, A Single Shot, in Vancouver with William H Macy and Forest Whitaker.

Talking about Cars 2, the sequel to the hit 2006 Pixar animated film, she's anxious to demonstrate that the process was no different from working on a live-action movie. "I mean I did a Woody Allen film and in a way it was the same experience. He doesn't encourage too much preparation or thinking about it before you turn up. That's just the way Woody works and so pretty much the first time you say it, is the first take and that's the last take generally. You got one shot. And while that's not the same with John [Lasseter], the preparation process is the same and there really isn't time and you have to sort of just use your own wits," she says.

"Its strange how similar the process is. With Match Point, after I got the script, I remember saying, 'I don't really want to do this.' And Woody said, 'I cast you in this film because you can do this, because this is how I see the part,' and so you just have to trust who you are is who they wanted, and so I felt the same way with this film, knowing that John Lasseter had specifically developed the character around me."

She even further likens the process to working with Martin Scorsese on Shutter Island. "The experience of working in those different jobs obviously is extremely different in each case but what's the same is that it's deceptive. It's so relaxed in each case because all three of those directors are masters of what they do, and so there's no feeling of anxiety from them about how I act almost to the point where you think, 'Why can't you just direct me and tell me what I'm meant to be doing?' But none of that goes on. There's a total lack of anxiety and they don't seem to be worrying. It's almost like they're there but they're not really there and then you look at the movie and of course every frame has their fingerprint all over it like they are the puppet master. They just know what they're doing."

Surprising even herself even at the ease with which she has adapted to her American lifestyle, she has lived in the States for more than a decade, finally becoming a US citizen 18 months ago.

"Because my husband is American, there's a certain perception among us Brits that he's probably a bit more stupid than we are. Then he has to prove them wrong," she says. "Whereas I, on the other hand, am at an unfair advantage here because people assume that because I talk with this posh English accent that I must be intelligent, which is not, by any means, necessarily true.

"He teases my mum about it all the time because she thinks she's not at all biased about Americans. I remember when we were getting married, she called him up on the phone and said, 'I was thinking we might have a boys' choir in the church when you get married. What do you think about that, Sandro?' And he said, 'Well, yeah, that would be great. We don't have anything like that in America.' To which, she says, 'Yes, I didn't think you did.' He was teasing her. Of course, you have the boys singing in church in America. So people don't realise they have preconceptions but, especially in England, I think there's a little bit of a presumption that everybody's something," she says of her husband who enjoyed a similar privileged childhood as her, attending Phillips Exeter Academy high school and graduating from Yale; briefly involved with another British actress Rachel Weisz before meeting Mortimer.

And as for the boys' choir? The couple put their own stamp on the wedding by hiring a Mexican punk band to perform at the reception.

The daughter of Sir John Mortimer and Penelope Gollop, she has two half-siblings by her father's first marriage although she and her younger sister, Rosie, were both surprised a few years ago to learn that they had an older half-brother from their father's affair with TV actress Wendy Craig. Incredibly close to her father, the actress admits she still has a hard time coming to terms with his death two-and-half years ago.

The irony of playing a chic sports car is not lost on Mortimer, 39, who, by her own admission, is a hopeless driver. "My first car was an old Saab. And then I had a BMW that my father-in-law joked he needed a tetanus shot to get into because it was just so full of old half-eaten sandwiches and bits of coke cans and things. But my husband doesn't allow me to drive anymore because I'm very unreliable," she jokes. "We live in New York and my husband refuses to get a car because it's just so nightmarish parking and I'm desperate for a car. I don't know why. I just really want one.

"I borrowed a friend of mine's car when they went on holiday to prove to him that it would be actually fine and within the space of a morning, I got three tickets and when I drove past into the back of another car the number plate fell off, I had to take the number plate with the car to a garage and get it screwed back on. The same time, I got ticketed for driving while I was on my phone; driving without a seatbelt and then for parking in the wrong place. So, yes, I'm banned from driving now."

If her children haven't been able to see much of her work, then she's found new kudos among her young family with Cars 2. "I'm pretty much a rock star in my seven-year-old son's eyes," says Mortimer when we talk at the northern California headquarters of Pixar animation. "I have a whole newfound respect from him. He's here today and just sort of almost overwhelmed to the point of tears on how amazing it all is. He's got so much credit just from being related to me. Because all his friends at school, they've seen trailers in the cinema and they think its so good."

In common with many actors, she enjoys taking her children on location with her, describing how her son had first-hand experience of one of Cars 2's comical scenes where tow truck, Mater, visits a typical high-tech Japanese toilet. "Almost the same thing happened to my son when I was a filming a movie in Tokyo two years ago and we arrived at the hotel, one of these huge sky-rise hotels, and he was peeing in the loo and he saw all these – he described it so perfectly. He said, 'I saw these buttons and I thought well one of them must be the flush I guess,' and so he pressed this button and then he described it like it was almost in slow motion. His arm came out like 'umm' and he's thinking, 'What's that and why are there all those little holes in it?' and just as he went 'Why are there all these holes in it?' it just sprayed and he comes back into the room, all wet, saying, 'Mom, I have a bit of a problem in the loo...' So that scene just cracked Sam up when we watched the movie together. It really resonated for him."

Another first for Mortimer is that she now has a toy car of her character. "I'm delighted to have a toy. I had so much pride. It's possibly the highlight of my career."

'Cars 2' opens on 22 July