Hermione Norris: The ice queen warms up

For her latest role, in the film Separate Lies, Hermione Norris plays Priscilla, a buttoned-up, meticulous legal secretary with an ice-blonde bob. Ideal fare, it would seem, for the actress who has made her name playing cool characters such as the publisher and yummy mummy Karen Marsden in Cold Feet.

In reality, Norris is no control freak: "I'm far too chaotic. I wish I could be that controlled and that smart and that my life required that of me but it doesn't, thank God! I've got baby sick or baby food all over me and I'm in jeans most of the time," she says. When we meet on a crisp, sunny morning on London's Portobello Road, her blonde hair is flyaway and she is dressed in a flowery skirt, grey military-style jacket and flat brown leather boots. The only touch of frost is her pale pink lipstick.

I ask her about the aura of icy calm she projects on screen. "You find that, after a while, this picture is painted of you and you almost would like to meet that person, because it feels like a different person from who you are," she replies. How about the description of her, in one paper, as the "queen of deadpan"? Norris guffaws: "Who wrote that? I take that as a compliment." It is dry wit that makes her laugh - "As arid as it gets."

Separate Lies is the directorial debut of Julian Fellowes, following his Oscar success as the writer of Gosford Park. The film hinges on the triangle of a frustrated housewife (Emily Watson), her lawyer husband (Tom Wilkinson) and an aristocratic bad-boy (Rupert Everett). Norris plays Wilkinson's faithful PA, "a woman in her late 30s, who is married to her job and in love with her boss, a lonely, isolated creature who cuts quite a tragic character." Norris nevertheless brings a touching humour to her role on the margins of the trio.

As one might expect from such a self-professed snob as Fellowes, the film flits between the City, Knightsbridge and the Home Counties. I suggest that it's not exactly a film for the masses. "I think it was bound to evoke this kind of response, a) because Julian directed it and b) because the story is set within a privileged world," she says.

"If you're viewed as one of the people, you're more readily taken into people's hearts. It's almost as though, if you have privilege, you are denied human experience. You can have grief, cancer, loss, broken relationships, divorce - but somehow it's undermined... It's naive and it's foolish because nobody is exempt from human experience."

Norris ran the gamut of human experience in Cold Feet. It's still the role she is most recognised for, although a stint as DCI Carol Jordan in the crime series Wire in the Blood upped her profile again.

She doesn't mind talking about Cold Feet, and she likes it when people talk to her as if she were Karen: "Maybe it's false intimacy - but for me it's quite nice, tragically." She identified with Karen. "We're all getting on with our lives and we're always trying to do the best we can and then suddenly bits explode out of you. That's been my human experience, I suppose. I thought it was very real."

While most of her co-stars have been in various TV vehicles, Friends and commercials, Norris and her on-screen husband David, played by Robert Bathurst, have kept a low profile. Does she think that people found it harder to warm to them? "I think it is a fact that people will always have less sympathy for people who have more." Of celebrity she says, "I find it frightening and overwhelming... I would rather maintain my family bliss and haven."

This bliss consists of her husband, TV executive Simon Wheeler, and their 16-month-old son, Wilf. She was back at work three months after the birth, but would do it differently next time. "My natural instinct screams out to me that I should be with Wilf. And yet I know I would go completely crazy if I didn't work. I feel constantly torn."

Norris has never known anything but performing. She went to Elmhurst Ballet School aged 11 - "every little girl's dream come true", but also "bizarre, a mixture of the army and a nunnery". Her training stays with her; she is poised and "still walks with her feet turned down". She found ballet too restrictive, so turned to acting and gained a place at Lamda. Her first job was playing Helena in A Midsummer Night's Dream in Colchester and her career since has been "a slow, methodical graft".

Now, she turns down more work than she accepts: "Television, dare I say it, is at a bit of a crisis point. Trying to find good writing is difficult." She has "come to the end of the road" with Wire in the Blood, and has filmed a thriller, The Kindness of Strangers.

When she is not working, she visits her cottage in Dorset and listens to music: "My husband always says I love suicide music, anything that's dirgey." Her religious faith forms "the cornerstone" to her life, although it has "waned in periods". She is 37, but looks forward to being 50: "I'd love to say that I'll grow old gracefully, with serenity and dignity, but when I watch my crow's-feet becoming like the Grand Canyon, I think, 'God, give me some cream!'"

For now, she exudes the Zen-like contentment of someone who has little left to prove. It was not always so: "If I used to go running, I'd have to run five miles a day and ended up having a knee operation. I went through a phase of doing bungee jumps and whitewater rafting." She sums up, in her trademark deadpan, "Now, frankly, I would rather sit down with a nice cup of tea. I've mellowed. Or, I don't know if I've mellowed, I've aged!"

'Separate Lies' is out today