I'm the real life Stacey

Page was nominated as Best Female Comedy Newcomer at the 2007 British Comedy Awards. She is just about to open in a controversial new West End play, Neil LaBute's Fat Pig. The irony is she is not a newcomer — she has been acting professionally for nine years, since she graduated from Rada in 1998.

For a long time she was stuck playing virgins in costume dramas — "I just looked so innocent and pure." But then she was cast as a porn actress in 2003's Love Actually.

When Corden and Jones were casting Gavin & Stacey, they remembered her from Love Actually and Russell T Davies's Welsh drama, Mine All Mine. "They told me later they wanted someone who was really cute and lovely and also a little bit dirty underneath," she giggles. "Stacey loves her family, but she's so feisty. She doesn't mind shouting at Gavin. And that's probably why I'm so like the character because I never stop talking and I don't censor what I say."

The irony is Page was originally sent to drama lessons because she was a shy only child. "My mother just wanted me to be able talk a bit louder at job interviews." She fell in love with acting, joined the National Youth Theatre of Wales, won a place at Rada and was snapped up by an agent in her first year.

Like Stacey, Page has risked all on love at first sight. She fell for her husband when they were both in the 1999 costume drama David Copperfield — Page as Dora, Thornton as Ham Peggotty. They never met during filming.

But when the drama went out on TV, she was smitten. "When they got to the bit where he lifts up the young Daniel Radcliffe playing David, and put him on his shoulders, I said to my mother: I want that man to be the father of my children.' He was so rugged and manly with this dark, curly hair." Unbeknown to her, Thornton was sitting watching the drama in his native Bradford, transfixed by her scenes as Dora. "He says he could feel this warmth coming off the screen and fell in love with me." They didn't meet for two years but her crush increased after seeing him in BBC1's Playing The Field. "I remember one scene he had to drive a motorbike and wear biker's leathers, and I was just: It's him again: he's gorgeous.'"

Eventually Page's friend Maxine Peake was in the National's production of The Cherry Orchard. "She phoned and said: I'm in a play here with this fella and he says he's in love with you.'" James shouted down the phone: Tell her it's Ham Peggotty!' So I thought: Oh my God, it's this man I've been watching for years on the telly and he likes me. Things like that don't happen."

She shot down to the National to watch the four-hour play. "Afterwards I went round to the green room to be introduced. He says he was totally shocked because he was expecting this very English girl with blonde hair down to her thighs wearing a pink ballgown. But I'd just been filming The Cazalets so I had this very short 1940s bobbed hair and trousers. And, of course, I was really, really Welsh."

At first Thornton was the more famous actor but today she gets recognised everywhere as Stacey. And now Page is about to shake off her girl-next-door image — playing a bitchy American office worker in LaBute's Fat Pig. "It's great because I get to be hard and strong in a business suit. It's the most grown-up I've ever been."

Fat Pig is the brilliant, coruscating tale of Tom, who falls for a plus-size woman, Helen. For the first time he feels real contentment — but can he withstand the cruel jibes of his coworkers and ex-girlfriend, Jeannie?

The play caused a sensation in New York. LaBute, who himself lost and gained 60lbs on the Atkins Diet, wants us to think about how we demean ourselves by harping on about weight. But the play is also funny. The London production stars Peep Show's Robert Webb as Tom, Kris Marshall as his best friend and, of course, Page as Tom's bitter, very thin ex-girlfriend.

Neil Labute is directing. Page was initially terrified. "Ella Smith, who plays Helen, and I were saying we expected him to be nasty to the two women in the room but he's so lovely and nice and funny." And Page has even ended up feeling sympathy for Jeannie. "At first I thought: She's a bit of a bitch.' Then as soon we started acting, I thought: Well, blinking heck, I can see why she's like this!' All she's asking for is a straight answer: Will you tell me, are you seeing someone?' Poor Rob, I'm constantly in his face screaming. Any PMT until September, he's going to be getting it."

There's a touching, down-to-earth quality to Page. She meant to diet for Love Actually but forgot. "Once I had rice and chicken instead of a McDonald's and I thought: Well, that's done it.'" Although she's teeny, she insists: "Why should I have a completely flat stomach, or worry about having cellulite or my boobs not being big enough. I quite like those human quirks. I don't want to look like a Barbie doll."

Most of all, she is broody. "I said to Rob Brydon: Well I've got a good 10 years left in me' and he went [she imitates his lugubrious accent]: No you haven't, I'm telling you.' Thank you, Rob, that's lovely. So I told James this morning: After the Gavin & Stacey Christmas Special, I'm having a baby!'"

Fat Pig is at the Trafalgar Studios, from 16 May. Information: 0870 060 6632.