Taped to Jane Horrocks's dressing-room mirror is a sheet of paper on which she has written the word "Anarchy". Admittedly, it's not a room that honours the principles of feng shui, but the worst you could say is that it looks mildly disorderly - a miniature skyline of "Good Luck" cards stands beneath the mirror, and I am sharing the sofa with a Wizard of Oz edition of Monopoly. By the sink is a bottle of washing-up liquid and a pair of Marigolds. Horrocks finds cleaning very satisfying. "I was on the Underground the other day and I saw a woman scrubbing a sign. She looked like she was really enjoying herself. I heard the 'snap' as she put on her rubber gloves, and I felt dead jealous."
In the corner looms a magnificent yellow orchid that is poised to colonise the room. It's a gift from Jennifer Saunders, who picked Horrocks to play the gormless Bubble in her sitcom Absolutely Fabulous. Bubble is perfectly named: you feel sure that if you tapped her with your fingernail, she would burst with an audible "Pop!" The woman who plays her, on the other hand, is robust. She is small and slender, but her handshake is guaranteed to weed out the feeble. Her Lancashire accent, with its shrill, sharp edges and its unexpected pockets of bass, sounds authoritative even when she's making chit-chat.
Horrocks points out that she has imposed her own structure and superstitions on this space. "There are certain things that need to be here," she says, gesturing to a white towel that belongs to her four-year-old daughter, Molly. "This has to be folded just so." And if it isn't? "I'm loosening up about it. I can see that if I rearrange the towel, then something interesting might happen tonight. It's nice to put in some danger when you're doing a long run. I've got to keep scaring myself. I've got to get my heart beating." It's evident that she likes to shake things up on stage, but I'm not altogether convinced she could sanction the disruption, the anarchy, represented by an irregularly folded towel in her dressing room.
The play is Stephen Poliakoff's Sweet Panic, in which Horrocks is Mrs Trevel, a prim parent who transfers her obsessive attentions from her son to the child psychologist treating him. The performance begins from a point of comic haughtiness, with Horrocks barking her lines and snorting like a prize thoroughbred. But as Mrs Trevel warps into a kind of stalker, the actress makes her righteousness comforting as well as unsettling. I'm conscious of the character's costume hanging on a rail by the door - a selection of frumpy patterned numbers, and a heavy black coat beneath which she barricades herself. Horrocks couldn't be dressed more differently, in her light blue blouse and jeans with Chinese characters printed on the pockets.
It transpires that Mrs Trevel is the reason for the "Anarchy" note. "She's on a mission to stir things up. She'll try anything." The actress has a list of phrases that she uses as points of concentration before each performance. She unfolds a sheet of foolscap and reads from it: "Tunnel vision. The client wanting to heal the healer. Play to win." We agree that the character has flashes of harsh wisdom, such as when she asks God to preserve her children from the curse of being original. It's a subject close to Horrocks's heart.
"Originality used to be embraced," she says, almost fuming. "Now it's considered a great sadness if you're an oddball. My son, Dylan, who's six, is quite shy, and that seems to have become a crime. Everyone wants to bring you out of your shell. Why can't you just sit and observe? Or be bored, if that's what you want to do?"
Horrocks was a shy kid, too. She was born in 1964, lived with her parents and two brothers in the Rossendale valley, and never said boo to a goose, or to anything else for that matter. "I always chose confident friends so I could hide in their shadow." She only got noticed during impromptu amateur New Faces sessions attended by children from all over the school, who crowded in to witness Horrocks's on-the-nail impersonations of Cilla Black, Shirley Bassey, Judy Garland - most of the performers, in fact, she would later mimic on stage in The Rise and Fall of Little Voice, which Jim Cartwright wrote for her and which was directed on stage by Sam Mendes, who was then her boyfriend.
"My friend used to compère at school," she says. "It was just being daft. But I liked the reaction I got. I wasn't a good-looking kid, at least until I discovered make-up, then I got a bit better. My friends went out with loads of blokes and I did feel like an ugly duckling. But I realised, 'Hang on, I can do this thing, I can sound like other people, and everyone likes me doing it.'"
Sometimes a little too much. I recall with a wince that Horrocks appeared on Des O'Connor's chat show back in the early 1990s, when he badgered her into the role of performing monkey. "I know, I know," she sighs. "I told him I didn't want to do Shirley Bassey, 'cos she was coming on the show straight after me. And then he got me to do her. I apologised to her in the hospitality room afterwards, and she boomed, 'Don't worry about it, I was shaving my armpits at the time.'"
A career in seaside variety could have been hers. She might have been the new Faith Brown. (Whatever happened to Faith Brown?) Anyway, Horrocks eventually enrolled at Rada in the same year as Ralph Fiennes, though anyone hoping for a story of the crumpled Northern underdog triumphing over the plummy toffs should look elsewhere. "There was a mix of posh and not-so-posh, and I was in the not-so-posh brigade," she says breezily. "If anything, the poshies felt that the rest of us got more attention. I like the fact now that not many people sound like me. But I hate playing a role with my own voice. Bubble is close, but she's exaggerated. Whenever I video the kids, I can hear my voice off-camera and it's like..." - she makes an obscene cawing noise like a crow throwing a temper tantrum. "There would have been no point affecting an RP accent. Then I'm just like everyone else."
Homogeneity appears to be one of Horrocks's greatest fears. There is the pride in not having buffed or modified her accent, and the outrage at what she perceives to be the social erosion of originality. She detects signs of this trend in the number of people who now aspire to be in the entertainment industry. "I've noticed on public transport that you always hear people talking about acting or being in the media," she says, with the conspiratorial air of someone gossiping over the garden fence. "You never got that 10 years ago. Now you're surrounded by it. It's because of all these competitions and reality shows on TV. They encourage people, so now it's ten a penny and suddenly you're not unique any more because so many other people are participating in what you believed was an original career choice." There is a touch of panic in her voice, as though someone is out to steal her identity. I glance at that orchid, and find myself distracted by memories of Invasion of the Body Snatchers.
This dread of being mistaken for everyone else has turned up in her work before, notably in her audacious and award-winning performance in Mike Leigh's 1990 film Life Is Sweet. Horrocks was a real shock to the system as Nicola, the bulimic back-bedroom rebel with a face like a clenched fist and a voice that suggested Donald Duck with depression. Her fingers snapped violently at the air as she squinted through her spectacles and pronounced that everything was "bollarks". And still she clung to the family that she professed to loathe. People always recall the scene in which Horrocks has chocolate spread licked from her naked chest, just as they witter on about that point in Macbeth when she urinated on stage. But the most pertinent moment in Life Is Sweet comes when Nicola's twin sister tells her plainly: "You can't leave, can you? You want to be with us."
There is the same tug-of-war in Horrocks. She tells me that her mother says she's like "the woman in the bottle of vinegar", but I confess this means nothing to me. "She was stuck in this bottle of vinegar," she explains patiently, "and she wanted to get out. But when she was out, she wanted to be back in the bottle of vinegar." Like "the grass is greener on the other side," I suppose, only less grassy and more... vinegary.
It transpires that her mother made this observation recently, when Horrocks was bemoaning the fact that while she feels tremendous guilt whenever she is away from her children, she could not contemplate forsaking work. She tried a few years ago, after suffering from the pressures of promoting the film version of Little Voice in America while pregnant, and then pushing an album she had recorded. "I just gave up for a while." She makes it sound like removing a hat. Perhaps that's how it felt. She admits that her appearances as Prunella Scales's terminally embarrassed daughter in the long-running Tesco television adverts have given her complete financial security. She says it has made her lazy, too, and that sometimes she asks herself: "Shall I just sit back on my sofa and enjoy the fact that I don't have to do anything?"
But that never lasts. You can see her antsiness even as she contemplates the idea. Her partner, the writer Nick Vivian, occasionally suggests that she should order the weekly groceries online, but she won't have it. She likes the jaunt to the shops - to Tesco, of course. It would be a clear breach of contract if she were ever snapped picking out a jar of gherkins in Asda, or checking Pink Ladies for bruises in Somerfield. She gives a grim, tight-lipped smile when I bring up an old interview in which she mentioned being in Sainsbury's, even though it dates to before the Tesco campaign. "I like Tesco," she says, recovering. "And no one ever recognises me there." It's not clear whether this is a good or bad thing. "Sometimes if I get a rude checkout girl, I want to say, 'Do you realise who I am? I advertise this joint!'"
At the end of Horrocks's two-year break from work, she felt frustrated. But there were clear benefits from becoming more involved in her children's lives, and not only for the obvious reasons. "The other mums are fantastic," she says excitedly. "I love socialising with them. It made me realise that housewives are interesting to be with, too." I ask if she had a fear of being categorised as just another housewife. "I think so. I wondered also if people would treat me differently because I was on TV. It was a huge relief when they didn't. I dress down at the school. No make-up. I look a real hound. The other mums are all gorgeous. They must think, 'Bloody hell!'"
Despite her apparent enthusiasm for this new-found assimilation, you can hear Horrocks trying to distinguish herself from the other parents. They are the glamorous ones; she's the plain Jane who can't be bothered with a bit of slap. In her professed dowdiness she becomes extraordinary again, outlining herself with a magic marker even as she gives the impression that she's fading into the schoolyard brickwork.
'Sweet Panic' is at the Duke of York's Theatre, London WC2; booking to 7 February (020-7369 1791)Reuse content