Catherine tate and I are talking about embarrassing crushes (are there any other kind?). "Ooh," she recalls, her face crumpling in memory. "It's that thing where you're not interested in someone until someone tells you they fancy you. And then suddenly you're 'Oh, right', you start making the case for it, don't you? And then when they go off you, you're left with this awful crush." Tomorrow Tate, 40, opens in the West end revival of David Eldridge's Under the Blue Sky, first seen at the royal Court in 2000 - a bittersweet comedy about the tangled love lives of a group of London teachers. Tate is Michelle, the hilariously promiscuous 38-year-old maths teacher who has just been dumped by the love of her life.
When I told friends I was interviewing tate, things went mad. Men flooded my inbox with sci-fi questions. My credibility soared with nine-year-old girls.
But her fans are worried. Isn't her career going to be a bit of an anti-climax after the nail-biting finale of Doctor Who last week, where her character, the Doctor's assistant, Donna, became the first female to turn into a half time Lord?
"For one brief moment I was the most important woman in the whole of the universe," she tells me. "Oh gosh, I can't thank Russell [T Davies] enough for just making that possible. For many people, I'm sure, what a gamble to take on someone like me who is known, by the vast majority of people, for wearing wigs and comedy teeth." It's easy to forget that tate, dubbed the funniest woman in Britain, is a proper actress. Since the Catherine tate Show first went out in 2004, she has become a national treasure - performing her "is One bovvered?" sketch in front of the Queen and playing mouthy schoolgirl Lauren opposite Tony Blair in that Comic relief Sketch.
But fans needn't worry, she is exactly where she wants to be - back on stage. Yes, she cut her teeth on the stand-up circuit but she also did three years at drama school and a year with both the royal Shakespeare Company and the national. She was terrific in the 2005 West end play Some Girls with David Schwimmer and did a stint in death row drama the exonerated.
"I thank God for my success on TV," she says, "because now theatre people say: 'Would you like to come and do a play?' When i went to drama school that was all I really wanted to do. You don't go there to learn how to act on TV and film."
On screen Tate is a chameleon presence - she can play anything from 15-year-old schoolgirl to 80-year-old gran. But in the flesh she is rather beautiful, with long auburn hair and a porcelain complexion.
She is naturally shy - she loathes being hugged and hates small talk. "I'm not frightened of a bit of silence." And she always keeps that voluptuous figure under layers (big boobs, she jokes, meant she could never run as fast as David Tennant in Doctor Who). But in Under the Blue Sky, she has to strip live on stage. The revenge sex scene with her nerdy teacher colleague in the play is graphic and funny - and very nearly erotic - until it implodes. The last thing she wants is for the sex to look actorly. "You can't be going: 'On this line, I will be undoing the third button of his trousers!'" Tate is in a long-term relationship (with stage manager Twig Clark), with a daughter, Erin, five, but she hasn't forgotten the full horror of London dating. We bond over the scene in Michael Winterbottom's film, Wonderland, where Gina McKee's character has to put her tights on after an awful one-night stand, then sobs all the way home on the bus to Elephant and Castle. We've all been there, I suggest. "That's exactly what it feels like at 10 to six in the morning," she agrees. In this new play she loves her drunken character Michelle, even though tate has never touched alcohol (she hates the loss of control). "She doesn't apologise for having slept with all these people. I think it's very honest and refreshing to have someone who goes, 'Yeah, I have a lot of sex', it just strips away the nonsense. She isn't just there going, 'I haven't got enough time to have a baby!' there's no question of her having given up."
Under the Blue Sky isn't a star vehicle. Tate was keen to do an ensemble piece and she shares the six-character bill with the likes of Francesca annis and Chris O'Dowd from the IT Crowd.
Considering she is such a powerful force in broadcasting, there is a real modesty
about her. She doesn't dress in a flash way, she always takes her mother to awards ("no one enjoys being Catherine Tate as much as my mother," she says) and she lives in a normal house in Mortlake.
Her mother's family have been florists for generations. Born Catherine Ford, she was raised in the Brunswick Centre, just off Russell Square. Back then it was a gritty council estate. "There was no Carluccio's and French Connection but there was always a sense of community."
Her mother left her father before she was born. She grew up - an adored only child - in a matriarchy. "It's never been a point of reference to look to a man," she tells me today. "It's not always a good thing, I suppose, but I think independence is always preferable to dependence." And yet the young Catherine was a worrier. She suffered from an obsessive disorder connected with word association which meant she was careful not to leave her jumper on the floor, since her mother's name, Josephine, also began with the letter J. But she knew she could make people laugh. "So I subconsciously made myself become the funny one so that would be my label rather than the ginger one or the red-faced one."
She attended Notre Dame high school in Southwark (she is still a regular churchgoer) and loved the theatre. It took four years of applying to study drama at Central School before she got in. After small theatre jobs, TV bit parts and a spell of stand-up, she was spotted at Edinburgh by a casting director who got her the role in the Dawn French sitcom Wild West, which led to BBC2 commissioning The Catherine Tate Show. The great thing about her writing is it is character-driven, rather than relying on boom-boom punch lines. She found her foul-mouthed cockney granny from visiting old people's homes. The neurotic mother she spotted at Peter Jones on the King's Road.
In many ways, it's a miracle the show ever appeared. It was filmed while she was suffering acute post-natal depression after an emergency caesarean. She didn't sleep properly for two years and felt incredibly guilty she couldn't breastfeed properly - until one night, at 2am, when she had lost all hope, a nurse came in and said: "You know what? In 10 years' time when she's at school, it won't matter."
Her partner, Twig, of whom she is very protective, gave up work to help with childcare. Today she says she has lots of friends who could not be more fulfilled being full-time mums. "But I don't think that would have been the choice for me because I like to work. Especially if you're lucky enough to do something you love. I will absolutely say that whatever job I was asked to do, whatever schedule I was asked to work, it is never going to be as hard as looking after a child."
Tate isn't a glass half full person - she jokes the glass isn't even what she ordered. She still worries incessantly - will Erin choke on her Rice Krispies when she's away filming? - and suffers occasional panic attacks. Astrology grounds her. Although she wishes she'd never talked about it now.
"I've become Mystic Meg," she laments. "That's one of the things David and I argue about, because he has no patience with it. But I'd say, well, it's more than a parlour game. I don't mean your stars in the paper - but I am interested in the alignment of the planets and the characteristics of the houses.
"I also know, categorically, unquestionably, when Mercury's in retrograde I'm always late, the trains are delayed and things don't turn out right. And that will happen twice a year," she tells me with wonderful, bleak satisfaction.
Tate is great if inscrutable company. You think you're getting on famously, then she closes down. "That's the poisoned chalice when you're shy, people assume you're arrogant," she admits.
She finds cheeriness exhausting. " Positivity is a great thing, but I genuinely can't communicate with people who START RIGHT HERE [she mimics, raising her voice]. You think: God, bring it down. Mainly because they're usually desperately unhappy. I admire it in one sense because of the sheer amount of effort - and I'm fairly lazy, so I couldn't commit to that kind of endurance test every day. But on the other end of the spectrum, I can't bear people who mope. Moping is horrible."
Her true friends are people who have lived through pain, and it has shaped them. "I've realised I need a gnawing, nagging, anxious doubt when I wake at 4am." But her fears go straight into her sketches. "In any situation - awful, good or average - there's always part of me thinking: 'That would be a good two-shot.'"
London is her passion - "it's in my bones" - and she loves her home comforts. She has a pathological horror of camping. "It's something I would never even consider," she shudders. "Have you heard of this thing called glamping? It's £7,000 for the weekend at Glastonbury so you can have hot and cold running water and a toilet. Go to a hotel!"
What's next? She's a big champion of new writing at the Royal Court and would love to do Shakespeare but is aware casting directors can be snobbish. "But you know, as long as I get the opportunities to work with the people in the industry that I admire, then I'm delighted to be, for 90 per cent of the population, the girl who said: 'Am I bovvered?'"
Under the Blue Sky opens at the Duke of York's Theatre, WC2, tomorrow. Information: 0870 060 6644