"When you are on the telly, it is an occupational hazard that people will go, 'You're off the telly, aren't you?'," she begins, a slice of wholemeal toast halfway between plate and mouth. "And that's basically the entire exchange. Mostly, but not always. I had this really nice chat, this most wonderful chat actually, with a guy on the tube the other day. I'd tripped over his feet, as I do, and he was embarrassed and I was embarrassed and then he couldn't stop looking at me. I didn't know if it was because he wanted to shout at me for stepping on his feet or because he thought he recognised me. Eventually, he asked me if I was me and I told him yes, and we had this really wonderful conversation about why he was on the tube, why he is retiring and what it means to be at that age to have both parents and grandchildren. And this was on the tube!" Here, the eyes become huge, and the eyebrows are all but lost to her hairline. "That's brilliant, isn't it? To sit on something like the tube and have such an intense conversation for three stations ... Well, I think it's incredible."
The toast, finally, reaches her mouth, and she takes a tiny bite.
"And that, as far as I'm concerned, is the only good thing about recognition. It brings people together." Now she frowns. "But I'm not suggesting that I, personally, am responsible for creating beautiful, cataclysmic reactions among the general public - of course I'm not. But I do like to think that being recognisable can help spark the kind of conversations strangers otherwise wouldn't have. The thing about people is that we are just so close, you know? People, all the time - so very close."
Here come the icebergs.
"I watched this fantastic documentary about icebergs the other night. It was fascinating. Basically, an iceberg is this mass of potential energy, a block of giant possibility, and yet all it does, mostly, is melt. If it cracks or turns over, or if a ship runs into it, it can explode and cause all kinds of havoc, but if it doesn't, then nothing happens at all. And sometimes I think that's what it's like to be alive, you know? Tube journeys are just carriages of potential waiting to happen, and most of the time, nothing does. And that's a shame."
She pauses, wrinkles up her nose and seems suddenly to take notice of our surroundings. It's 9.30am on a damp Wednesday and we're seated in a café alongside a gaggle of young mums and laptop-bearing young men having breakfast. The cappuccino machine is roaring, a baby is crying, and toast is being busily buttered.
"Sorry," she says, now self-conscious. "Am I talking a right load of old bollocks here?"
She thinks perhaps she is, and endeavours to explain why. She doesn't do many interviews because, as an actress, "it's my job to be other people - I feel weird being myself in front of you". Sometimes, she says, she can go off on tangents and doesn't know how to find her way back again, even when she has something to plug, like now. We are here to talk ostensibly about some Shakespeare she is doing in Stratford-upon-Avon, but this charmingly wayward woman won't quite stay on topic. She asks me as many questions as I do her, and at one point warns me not to misquote her - "I remember everything I say" - and begs me not to be a "bastard" when I come to write this up. She picks up her toast but changes her mind, and puts it back down again.
"So, what do you want to know?" she asks, with mock seriousness. "Because I don't really do much. Basically, my job is to look surprised on the telly. That's all. It's no great talent, if you really stop and think about it."
For the past few weeks now, Tamsin Greig has been looking exquisitely surprised on the telly because Green Wing has returned to Channel 4 for its triumphant second series. Green Wing remains the most deliciously surreal programme on the box, a warped comedy set among the corridors of a hospital in which patients are nothing but a minor aside in the dysfunctional lives of its erratic, certifiable staff. Among the weirdos and wackos is Greig's character, Doctor Caroline Todd, a comparative oasis of kooky calm amid the chaos.
"I suppose you have to see the world through somebody's eyes in the show," she reasons, "and that's what Caroline Todd is there for."
Greig's character is the kind of woman who is hapless at work and hopeless in love, all teeth and bed hair, and with her blouse tucked unwittingly into her knickers. If there is a door in front of her, she will walk into it. She is, I tell the actress, absolutely adorable.
"Really?" she says. "But she's annoying, too, right? She's a right pain in the arse."
Green Wing has been a deservedly big hit with audiences and (most) critics. Last year, Greig picked up a Royal Television Society award for her performance, as well as a Bafta nomination for Best Actress. But its success has also prompted an awful lot of flak. f
"I can see why some people are undecided," she muses. "It's a weird show, really weird, and many have accused it of being style over content, that it simply relies on technical jiggery-pokery. But, oh, I think it has such heart, such a soft and beautiful heart at the centre."
She does admit, though, that even she blanches at, as she puts it, the "sex stuff". In the first series, a 49-year-old harridan slept with a young doctor whom she realised only afterwards was in fact her son. "And then there was the toe-fucking incident," she whispers, cheeks reddening at having uttered the F-word. This is a reference to - well, to the toe-fucking incident (and we've not enough space here to go into that one). "I'm rather glad my mum and dad don't have to watch it because I think they would have been quite embarrassed."
And why don't they watch it?
She throws me a harsh glare that's only slightly ironic: "Well, probably because they are dead," she says.
Greig has been a professional actress for 15 years now. Her first role came in 1991, two years after completing a drama degree at Birmingham University, when she landed the part of Debbie Aldridge in Radio 4's long-running soap opera The Archers. While this sustained her for much of the following decade, she also did a bit of theatre and made intermittent forays into television: there were bit parts in the crime detective series Wycliffe, in The Lenny Henry Show, and in adverts for Diet Coke. In 1997, she starred alongside Dylan Moran and Bill Bailey in the Channel 4 comedy Black Books, and after the career-defining success of Green Wing, David Renwick, the creator of Victor Meldrew, wrote the feelgood BBC1 sitcom Love Soup especially for her. It has been pointed out to Greig, rather uncharitably, that she has simply played the same character over and over again. Each is put-upon and dizzy, each beguilingly silly.
"Yes, I suppose you could say I've been typecast," she considers, "but I hope not too much. In one sense, I guess, I could carry on doing this kind of work for ever, couldn't I? At least until everybody loses interest in me, anyway. It's comfortable, it's regular, it pays the bills ... Perhaps," she adds, quizzically, as if the idea is only just now beginning to dawn, "perhaps that's why I've taken this latest job."
This latest job, as she would have it, is a role in the Royal Shakespeare Company, in which Greig will, at last, fully stretch herself as an act-or. When her agent first came to her with the proposal, she was highly reluctant.
"I said no, initially, not just because I always say no initially, but largely because I've got three children, and I didn't want to miss their breakfast every morning. But my husband [the actor Richard Leaf] really encouraged me to do it. He said that's what actors do, they follow the work. But it was my mother-in-law, I think, who put it best when she said, 'If it looks impossible, it's probably precisely what you should do.' "
So it's her fault?
She grins. "In a manner of speaking, yes."
At the end of this month, then, Greig will exchange her sleepy north-London suburb for Stratford-upon-Avon and the Bard. When one becomes part of the Royal Shakespeare Company, she tells me, one doesn't just perform a single play and then return home again; one performs several - or, in her case, two. The first is Much Ado About Nothing, in which she will play Beatrice. And for the second, she will be Caroline in the rarely performed King John, which she describes as "one of Shakespeare's more political and serious efforts". She'll be away from home for nearly four months.
"It's wonderful, of course it is," she says, stress lines forming, "but my great worry is that I haven't been on stage in front of a live audience for 10 years now, so I'm going to have to make a big effort to get my confidence back. That might be difficult, because I'm a terrible giggler. Right now, I'm awfully nervous, and I'm worried that I'll really miss my family. But I'm grateful for the opportunity too; grateful and stunned. Why on earth they offered me the part in the first place, I'll never know. But then every time I get offered a new job I can't quite believe it. The trouble with me," she says, shrugging, "is that I permanently doubt everything."
In the manner in which she says most things - a combination of doe-eyed confusion and blushing embarrassment - Tamsin Greig readily acknowledges that she has been "very looked after" in her career. Had The Archers not come along as swiftly as it did, she wouldn't have been acting today. Why? She'd never have had the confidence to stick it out.
"Every drama school in the country turned me down," she points out, "and so I was lucky to study [drama] at all, even if it was at just lowly Birmingham University. But even when I came out with my degree, my mother promptly insisted I go straight to secretarial college to have something to fall back on, just in case - which didn't exactly fill me with confidence."
She felt drawn towards performance in the first place, she says, "because I was the typically annoying middle child, always showing off, and I thought that to get into acting would be a good way of turning something rather unpleasant into something productive." And yet, given her success rate to date, the 38-year-old will never confess to anything as ugly as ambition.
"I don't even really know what ambition is, to be honest," she says, "but I do have a certain faith in my - well, my abilities, I suppose you could say. But only recently. It's like when you first get into a relationship. The first time somebody says to you, 'I love you,' you don't entirely believe them, do you? Well, I don't. It takes time - years - before you build up enough trust in them. The same with my career, really. The first time somebody told me I was good, I didn't quite believe them. Now I sort of do."
A dreamy look passes across her face.
"But I also believe that, in a bigger sense, we are all looked after, really."
"By God, of course."
I decide not to challenge her on this notion that God is busy securing her acting parts while allowing war and starvation to wreak havoc upon the lives of countless others (we don't have the time, and I can't quite muster the courage), and instead ask about her future. Her lack of driving ambition suggests that, unless Richard Curtis comes calling, Tamsin Greig won't necessarily make the jump into films, and certainly not the kind they make in Hollywood.
"Oh, nobody would ever want to know me in Hollywood," she says, with her typical Who, me? insouciance. "I'm far too puffin-faced for that, too weird-looking. No, I think I'll probably stick to telly, if telly'll have me, though I wouldn't mind doing radio plays as well. I always did enjoy radio [her character in The Archers has disappeared off to Hungary, though the door has been left open for her to return]. Otherwise ... well, I don't really know what I'd do."
She takes a belated bite on her toast, which must be stone cold by now. It's been sat in front of her, largely ignored, for almost 90 minutes.
"I did used to like trampolining, but I'm probably past it, I think. You need to have a really strong pelvic floor to be good at trampolining, and I've had three children."
She is laughing now, Green Wing's Caroline Todd written all over her face.
"I've lost my whoopsie altogether," she laments. "I shall never trampoline again."
'Much Ado About Nothing' opens in Stratford on Thursday; www.rsc.org.ukReuse content