"I used to call them my monkeys who sit on my shoulder,” Connie Fisher says earnestly. “One's a good monkey, one's a bad monkey. I shoot the bad monkey. My last director used to say, 'I'm going to shoot the monkey!'”

That was Jeremy Sams, on the West End production of The Sound of Music, in which Fisher won herself the part of Maria through a high-rating BBC Saturday night talent show in 2006. It's not clear which monkey Sams had in mind. Good, it transpires, is her “very confident” alter ego; bad is “self-doubtful”.

Self-doubt isn't something we would readily associate with Fisher. Her poise was clear to millions of TV viewers and she bade farewell to the stage role of Maria on February 23 this year, after a punishing 18-month run.

In her new show she plays Sonia Walsk, a kooky New York lyricist who hears an ensemble of extras in her head in Neil Simon's 1979 two-hander They're Playing Our Song. Walsk is based on Carole Bayer Sager, who wrote, among a number of other improbable things, the James Bond theme Nobody Does it Better, with the equally garlanded composer Marvin Hamlisch. He is represented by the character Vernon Gersch as Sonia's fractious partner in Simon's oddball comedy, which describes the pair's romance and features pop ballads by them.

The 25-year-old Fisher is now based in the Menier Chocolate Factory in South London, where she'll be performing to a modest audience of 170 a night. She's in determined, if guarded, good humour, but neither of her monkeys will stay reliably dead. Personally, I have more trouble with the good one. “This was the perfect thing for me to do next,” she says brightly. “Alistair [McGowan, who plays Gersch] is so funny; a brilliant musician. I really wanted to do something non-commercial in an intimate space.”

Except that what she initially signed up for next was a Simply Bernstein concert at Cadogan Hall in Chelsea in April, and a Connie Fisher tour, due to open at Blackpool Opera House on May 31. Both were cancelled in February, the latter owing to poor ticket sales.

She stiffens slightly at the mention of this. “Playing a role is much more exciting than just being Connie.” She insists: “I can do a concert tour any time; this is an opportunity I would have missed. I want very much to prove that I'm not a one-trick pony. That there are more sides to this crystal. Maybe you won't like it. Maybe you will. But I like a challenge and this is a challenge.” She pauses, aware that this is not an entirely convincing argument. “I was excited about the Bernstein,” she says, softening. “We did the photos and everything. It was going to be great, then all of a sudden it was off. But that's the way in this business,” she recovers, smile firmly fixed. “Opportunities are like buses: there's always another one coming.”

The thing with buses, though, is that they tend to all arrive together, and then, infuriatingly, none arrive at all.

Fisher has always told a story larger than her own. The telesales girl from South Wales trained on a full scholarship at Mountview drama school, and then failed to land a single professional role until the TV opportunity in 2006. Her victory was widely hailed as one of talent and authenticity over bland good looks and professional polish.

On November 15, the West End show opened to universal critical acclaim for Fisher's fresh, vocally thrilling performance. But four months into the run, she burst a blood vessel in her throat and lost her voice. Suddenly, Fisher became the poster girl for another morality tale altogether: pushed far too far, way too soon. She returned to the stage two weeks later, though she initially relied on recorded “click tracks” to hit her top notes. As aspiring Josephs and Nancys arrived to claim the public's adulation, Fisher's share of it never quite recovered, casting doubt on the claim of audition shows to uncover bona fide stars with lasting career prospects.

Now out of her nun's habit, where Fisher treads, Lee Mead (the winner of Any Dream Will Do) and Jodie Prenger (I'd Do Anything) are likely to follow. She knows that she is the audition-show guinea-pig but is stalwart in her defence of them. “Of course I'd rather have got there in the traditional way,” she says, “but people who say the shows are a bad idea are just gutted it wasn't their ideal role. I feel very lucky.”

Through the process, she dropped two dress sizes from stress (though she's now back to a healthy size 12) and had aspersions cast on her talent and on her personal and professional relationships. Soon after The Sound of Music opened, a tabloid newspaper ran the story that she'd been unfaithful to her long-term partner, Neal Williams, with a sound engineer from the show. She now lives with Williams in Finsbury Park, North London, and the rumours, she impatiently maintains, are “all rubbish”.

The Daily Mail then published another, that her co-star Ian Gelder had crowned her “the most unpleasant, unprofessional newcomer” of the show.

“It's not true,” she says firmly. “I got on well with the principal cast. I learnt a lot from them.” Though she adds: “Ian Gelder in particular is very old-school, a cool guy, but some people are old-school, and some people are willing to move with the times. This is the new way of casting and I want to be part of it. It has hardened me, but get thrown in at the deep end [and] you learn to swim pretty fast.”

From telesales to eight West End shows a week seems a touch too deep. And there's no let-up. Aside from the Menier run, she's released one album and has another in the pipeline for release in the autumn, when she'll also be appearing in a new ITV drama, as an Elvis-obsessed traffic warden. But her continued friendship with Lloyd Webber, whom she describes as a father figure, is perhaps the only unequivocal boon of her Maria journey. Fisher's relationship with her own father was blighted by his leaving the family when she was 17. “He's not my friend,” she says, “but he is my dad. He wasn't around for a long time, and he's only vaguely in my life now. I like being part of a triangle - my mum, my nan and myself. I think it makes me a stronger woman.”

Fisher reflects that she's never really happy unless she's making herself a stronger woman, climbing the next impossible mountain. She's had a recurring dream all her life that she's wading through thick mud. “I always feel the need to be better than I am because I feel half of me is missing, perhaps,” she says, referring to her twin brother, Justin, who died at birth.

But shouldn't she now stop indulging this need and enjoy her success? “I don't feel like that any more, about having to prove myself.” But ... she just said that she did. Not a one-trick pony, more sides to the crystal ... “That's my self-doubt monkey talking. Put another way - I have a need to show this other side of me off. That's the confident monkey.”

At one point during the interview I went off to get her a menu. Playing back the tape, I find two minutes of Connie Fisher minus both of her monkeys. The 25-year-old actress who worked hard to earn herself a break sang a quiet little melody at a restaurant table on her own. She sounded very beautiful indeed.

They're Playing Our Song by Neil Simon runs at the Menier Chocolate Factory, London SE1, July 25-Sept 28. Box office: 020-7907 7060

Living the dream: five winners ... and where they are now

Lee Mead (Any Dream Will Do, 2007)

The “People's Joseph” is treading the boards in the West End. Advance sales exceeded £10 million.

Paul Potts (Britain's Got Talent, 2007)

The 37-year-old tenor won the public vote with that old classic Nessun Dorma, from Puccini's opera Turandot. He has since toured the world and sold more than three million records worldwide.

Danny Bayne and Susan McFadden (Grease Is the Word, 2007)

McFadden bade farewell to Sandy last month to star in a touring production of Seven Brides for Seven Brothers. Bayne is still belting out Danny in the West End.

Jodie Prenger (I'd Do Anything, 2008)

Rehearsals for I begin in September. Prenger will perform six of the eight weekly shows at the Theatre Royal in London from December.

George Samson (Britain's Got Talent, 2008)

Samson, 14, from Warrington, has been delighting fans around the UK in the BGT live tour and will appear in the Royal Variety Performance in December.

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