Arabella Weir: Does my neurosis look big in this?

Arabella Weir has made a career out of her insecurities. And, she tells Julia Llewellyn Smith, there are plenty more where they came from

Arabella Weir is fascinated by my Diet Coke. "Oh God," she exclaims. "Can I have a sip? It's so delicious, but I can't allow myself one. No, no! Only sparkling mineral water. I had my cup of coffee this morning. Oh, that man's having a glass of wine with his lunch," she says, looking round the Pizza Express in central London where we're sitting. "Isn't that heaven? But I can't! I'm doing a voiceover this afternoon. I'm sorry, I'm picking my teeth – no don't write that!"

Arabella Weir is fascinated by my Diet Coke. "Oh God," she exclaims. "Can I have a sip? It's so delicious, but I can't allow myself one. No, no! Only sparkling mineral water. I had my cup of coffee this morning. Oh, that man's having a glass of wine with his lunch," she says, looking round the Pizza Express in central London where we're sitting. "Isn't that heaven? But I can't! I'm doing a voiceover this afternoon. I'm sorry, I'm picking my teeth – no don't write that!"

Best known for her comedy sketches on the male-dominated The Fast Show, Weir specialises in female neuroses. She invented the characters Pushy Saleswoman (No Offence), Girl Men Can't Hear, Different With Boys and, most famously, Insecure Woman, who – even when reporting live from a war zone in a flak jacket – wondered "Does my bum look big in this?" A novel of the same name was a chick-lit bestseller, and the catch phrase became so popular that Weir now threatens to sue advertisers who purloin it.

Her third novel Stupid Cupid is published this month and in the autumn she will star beside Richard E Grant in her own sitcom. Yet, despite such successes, Weir is still is comically similar to the fictional creation who made her name. "You're thin," she accuses me on several occasions. So are you, I retort truthfully. "No, I'm not! I decided this morning. I'm 'slat'. Slim and fat at the same time."

Years of therapy have made Weir, 43, clear about the roots of her low self esteem. The daughter of a diplomat, she had an itinerant childhood. "I lived in these amazing places like Cairo," she explains in tones as crisp as iceberg lettuce. "But it's not a good life for a kid, changing countries every two years. If it had been a toss up between that and a stable home life in London, I'd have gone for the latter."

When she was 10 years old, her parents embarked on long and difficult divorce proceedings and she was sent to live with her father in Bahrain, which she took as a form of maternal rejection. Now, she says, she gets on well with both parents, but at the time they made her feel she was neither attractive nor clever enough. At Camden School for Girls in north London, she indulged in bouts of overeating and undereating, and decided to be "the class show off and popular". When her mother got a teaching post there, Weir told her to fuck off in class. "Now I think of my own daughter doing the same to me and I shudder," she says.

Weir went to drama school in London and for the next decade struggled as an actor. Until the age of 37 she never earned more than than £10,000 a year. "Like most actors, I was having no great success or recognition – which is what I needed if that void inside me was to be filled. For someone who's as insecure about their body as I am it was a demented choice of career. It meant I was absolutely fixated, all my prospects depended on how I looked."

Her personal life was equally tempestuous. "I felt I was destined to go out with tossers – not that they were all tossers, but there weren't too many gems. My first big love turned out to be a heroin addict," she says cheerfully. "It was probably because of him I didn't go to university: I was so insecure about relationships I was too frightened to leave his side to go and live in another city. That was the problem, I had no idea of who I was or what I wanted. I just let myself drift. I'd tie my daughter to a wild horse before I let her do that."

Everything changed in 1994, when Weir joined the ground-breaking The Fast Show. "It was a phenomenon, but there were still times when I felt like the token girl," she says. "A friend once asked me if I knew Paul Whitehouse. I said 'No' as joke, and he said: 'No, I didn't think you would.'"

Personal acclaim came two years later with Does My Bum Look Big in This? "It was the most successful book of its kind after Bridget Jones and that meant more to me than The Fast Show because it was all mine," she says. "All these women wrote to me who had no notion of the television character and said 'God this is me!'"

The success brought her not only financial independence ("I no longer had to do the shitty jobs that are the lot of most actresses my age"), but emotional catharsis as well. "By outing my own insecurities, they no longer frightened me," she says. "I think that's why men don't understand female fiction, they just see neurotic characters as sad, but women feel empowered by sharing information about their lives."

In her latest novel, Stupid Cupid, the heroine carries on making wedding plans after she has been dumped by her dull fiancé, in the hope he will change his mind in time for the big day. "All that definitely comes out of personal experience," she says. "I used to think that a big wedding, with a lovely dress and a cake, would make someone who was only OK perfect. If someone had a grand 30th birthday party and said: 'This means my thirties will be a wonderful decade,' you would think they were mad, but for a long time that was what I thought a wedding would do for some imperfect relationship."

Weir is happily unmarried to Jeremy Norton, a 31-year-old fish parasitologist. They met when she employed him to redecorate her study in Crouch End, north London. "He arrived on Monday, we got it together on Friday, within a few weeks we were in love and he never moved out." Seven years later, the couple have two children, Isabella, four and Archie, two.

"It all happened rather later than I'd have liked, but by the time I met Jeremy I'd finally achieved professional success. Before, I wouldn't have allowed myself to go out with someone like him, who was much younger, a student. Before, I thought of men simply as providers and husband material. Now I know it's more important to find a good mate to share your life with. Ideally you'd have both, but Jeremy's a scientist, so we're probably never going to be driving around in a Jaguar. Still I easily earn enough for both of us."

Having waited so long for recognition, Weir is refreshingly excited about its perks. She shows off her new clothes, given to her by Marks & Spencer, and jabbers happily about visiting the Caribbean for Holiday ("I bloody earned it, last time I got Cumbria"). This autumn she will tour with The Fast Show: "Four boys and me on a tour bus, very intimate, although contrary to popular belief I have never been intimate with any of them in any other way." Before that, she will star in an Alpen commercial, specially written for her persona. "It's the apex of my career – I'm only half joking because to have a commercial written for you means you've succeeded in your field."

She is distracted by the woman at the next table. "Oh! I like her trousers but you'd need a very small bum to wear them." I laugh, thinking she is joking, then I realise Weir is deadly serious.

'Stupid Cupid', Penguin, £6.99