It is the day after the Olivier Awards ceremony and it appears that Samantha Spiro, winner of best actress in a musical category, has come down from cloud nine. "You've caught me just before doing my hoovering," says the 41-year-old mother of two.

Twenty-four hours previously she was holding back the tears as she accepted her honour for playing the lead in last year's Regent's Park Open Air Theatre revival of Jerry Herman's Hello Dolly. It was the second Olivier - one of the most prestigious theatre awards on offer - of her career, and one of three won by the show.

The production delivered that thing for which every lover of musical theatre yearns but so rarely gets. It happens when melody, energy and story combine to create moments of theatrical ecstasy. And in director Timothy Sheader's staging it happened, not once, but twice. There was the scene where the red-coated waiters dance to the title tune; but the first really special moment came with Put On Your Sunday Clothes, for which Spiro and the cast pretended to be a steam train twirling their parasols like wheels. And then - a masterstroke, this - smoke rises out one of the passengers' top hat like it was an engine's funnel. Even some critics spontaneously applauded.

This was the number that Spiro performed on the night of the awards, in front of the celebrity-laden audience, after which she had to make a quick change out of her Sunday Clothes costume and into her blue silk evening dress to collect her gong. "As they announce your name as the winner your heart is beating so furiously, you can feel the blood pounding in your head. I hadn't experienced it before. I just felt really, really happy."

Except Spiro had experienced it before. Ten years ago she was a surprise casting choice (she says so herself) for the Donmar Warehouse's revival of Sondheim's Merrily We Roll Along. This was the show that led to her first Olivier Award.

"I used to think of myself as someone who wouldn't be doing musicals of any sort," says the north-west London-based actress. "I wanted to do classical theatre and gritty modern plays. I didn't see myself as being all eyes and teeth."

But then, presumably, neither did Spiro see herself as a busty cockney with a machine-gun cackle for a laugh. Looking back, her Barbara Windsor in Terry Johnson's play Cleo, Camping, Emmanuelle and Dick, was probably her big break.

Yet for all the ambition for classical theatre - most recently she played Maria in Twelfth Night with Sir Derek Jacobi - Spiro is, whether she likes it or not (and no actor is happy to be pigeonholed) one of this country's musical stars. "I've always enjoyed putting a song across but never considered myself as a real singer. But doing more I've started to feel more accomplished," she acknowledges.

It is all a question of choosing the right role. It has to have a lot of energy and a lot of comedy, she says. Before Dolly, there was her equally acclaimed Fanny Brice in the Chichester production of Funny Girl. Both roles have gallons of comedy and energy, and each has attached to it the ghost of past performances by Barbra Streisand. Yet reinventing Dolly and Fanny held no fear for Spiro.

"I never feel that sort of pressure. If people come along and see you and want to make comparisons then of course that is absolutely fine. But it wasn't something that was restricting me at all. You have to approach it as you would a Shakespearean role. You can't go: 'Oh I can't play this because Judi Dench or Maggie Smith has done it'."

There must have been something of that self-belief 30 years ago when during a school trip to the Regent's Park Open Air Theatre, the venue of her most recent triumph, Spiro sat watching Shaw's Androcles and the Lion. If there was a moment when she decided to act, that was it. "I thought, that's what I want to do. From literally that point there was never anything else."

There was little Spiro family history in showbusiness to encourage her. Her father wanted to be an actor, but became a property dealer. But the acting gene can be traced to Spiro's Polish-born paternal grandfather, Morris, who used to tour Russia with his one-man Yiddish show. Though he died when Spiro was just six, she remembers that she recognised in him something of her own performing talent. Or perhaps it was he who recognised the talent in her.

For Spiro's thank-you speech at the Oliviers, she told the audience of peers and producers all about the school trip to Regent's Park that triggered her career. But a little less gratitude and she might have asked the assembled great and good of British theatre how it is that a production acclaimed by the critics and loved by the audience fails to get a transfer into the West End. The same thing happened - or rather did not happen - with the Chichester Funny Girl. It must be hugely frustrating to be in a show that is so good and yet still does not get into the West End, while lesser shows seem to waltz in with no problem.

"Tell me about it," says Spiro. "It's a funny old thing really. I just have to laugh at it and go: 'You know, maybe the West End's just not for me'." This is not true, of course. It is just that Spiro appears admirably well-adjusted about it all.

"I've been so lucky to play the parts and sometimes it's just lovely to tie these little experiences up with a ribbon and put it somewhere in your memory bank and say: 'That was marvellous'. That's the only way I can think about it, otherwise you get disappointed that it didn't happen, when in fact it did happen and it happened magically." Then she pauses for a reality check. "This is me putting a positive spin on it. Every fibre of my being would love to get a call saying Dolly is going in. But I'm not one to sit around feeling disappointed."

It is all down to the unknowable calculations producers make to see if a show will make or lose money. And for some ridiculous reason, Dolly, with its fervent Jewish and gay following, never made the grade.

Coming up for Spiro is another Jewish role in Arnold Wesker's Chicken Soup With Barley. The classic is due to be revived at the Royal Court in London, though no date has been set. It would be the latest Jewish role since 2006 when Mike Leigh cast her in his "Jewish" play, Two Thousand Years. And she has also just finished filming a new comedy for BBC2 written by Simon Amstell and Dan Swimer, based on Amstell's family. "It's set in Gants Hill and its like a Jewish Royle Family," says Spiro who plays Amstell's Aunty Liz.

"Since being outed with Mike Leigh, it's been Jewish part after Jewish part," she adds. "I'm loving them. Absolutely loving them."