'I'm not like Claire'

People always ask, "How do you feel about death? Do you want to be buried or cremated?" I'm like, "I don't want to even go there: I'm 26 - I don't feel

'Everybody always says, "Oh, she's so me!"' Ambrose remarks in her off-hand Connecticut accent. 'Because Claire has all these great quips the writing team give her, and everybody wishes they could be that clever. But really she's had a very difficult life, growing up in a funeral home as the youngest sibling, literally half the age of her adult brothers, and kind of neglected for it, kind of an afterthought.

She's had to figure things out for herself. I'm definitely more hopeful than she is. And older. And married.'

And, right now, over here. The 26-year-old is using her annual hiatus from the surreal TV show to make her debut on the British stage. She opens this month in a long-neglected Sam Shepard play at the National Theatre.

In Buried Child she takes on another

Vince's mad Midwestern family make the Fishers look downright functional: Vince's grandfather fails to recognise him, his uncle has sawn off his leg by mistake, and incest has ravaged the family. 'It's just a very strange homecoming,' is the way Ambrose sums it up. But its British director, Matthew Warchus, claims it is Shepard's greatest play --as grotesque as a work by Mike Leigh, as Biblical as an Arthur Miller.'

He knew Ambrose was perfect for the part the instant he auditioned her. But she admits she is still struggling to make sense of it. 'Sam Shepard describes it as a comedy with a very dark title,' she observes. 'I guess there are funny moments, but from my character's point of view she's mostly terrified. I still don't really understand what's going on.'

Fresh from rehearsing a fight scene in the second act, she looks younger than her age in flared jeans and a thin, sleeveless green top. She walks in to the National's press office brusquely, as if she is verging on a bad mood, and throughout the interview swings between smiling charm and faint ungraciousness. Her skin is Persil-white, and you notice her pale ginger eyelashes, ruthlessly clean of mascara. She hails from a long line of Italian redheads on her father's side, the D'Ambruosos, who settled two generations back in New Haven, Connecticut. 'I always liked my hair,' she says casually. 'I think redheads are only one per cent of the population.'

Her father, Frank, is a cook and her mother, Anne, an interior designer. Her younger brother, John, is studying physics at college, and Ambrose grew up singing in the Catholic choir. 'New Haven is where Yale is, so there were a lot of opportunities to study singing and acting.

I would sing at weddings and funerals during high school. I always wanted to perform.'

She could have become an opera singer, but felt it was too big a commitment - 'I have friends who are up-and-coming opera singers and you have to be so devoted, like a ballet dancer.' So she went into acting. She had her stage debut at 14 in New York and made a handful of appearances in the TV shows Law And Order and the dark teen drama Party Of Five. Later she played Florence in the 2000 horror spoof Psycho Beach Party before landing the part of Claire - to get the role she had to pretend to be hearing the news of her father's death crashing the family's new hearse while tripping on crystal meth.

'TV's such a weird medium,' she says of the show. 'It's written quickly, produced quickly, on the air quickly, and then it's over. But Six Feet Under is probably closer to the end of its life than the beginning, and I want to do more theatre. It was a miracle that this play fell into my hiatus. Actually it overlaps a bit with the fifth series in the fall, but the producers have been very kind and let me do it.'

For three years Ambrose has been married to an internet entrepreneur-turned portrait photographer called Sam. The couple shuttle between their West Hollywood apartment and a house in the woods in the Berkshires, Massachusetts. 'It's our anniversary tomorrow,' she confides, 'and Sam's arriving today. I thought I'd book a pod on the London Eye.' I ask what he's like, and she flings wide a Persil arm. 'He's the anchor in the storm. He's a very good calming force in this life, with all this uncertainty of work, and jobs.'