Whatever guise she appears in - deeply serious as a desperate mother in the BBC's award-winning drama Suffer the Little Children, or jauntily absurd as Bubble, Jennifer Saunders's cretinous personal assistant in Absolutely Fabulous - Horrocks always cuts a dash. At the moment she is doing this as Lady Macbeth, in an audaciously modernistic, mobile phone-toting Greenwich Theatre production by Shakespearian wonderboy Mark Rylance. She was getting fed up of being offered neurotic battered wife roles - "I thought I'd rather be the batterer" - and having been quoted as saying Shakespeare was "boring", she wanted to prove it didn't have to be. "We've had a lot of bumptious people saying 'How can you do this to Shakespeare?' " she observes, quite piqued. "But if theatre's not experimental, it's dead: there's no reason to do it."
Jane Horrocks does something very experimental in the course of this production. She urinates on stage. "People haven't really been able to handle that," she says with a smile. Has she herself been able to handle it? People find it hard enough to produce a sample for the doctor, let alone on-stage in front of three coach parties of A-level students. "Sometimes there's only a little trickle and I think oh, that's a bit of a boring old piss tonight [assumes reviewer's voice] 'a disappointing piss'. But sometimes it comes flooding out like a horse, which is excellent, although the woman playing the gentlewoman gets a bit splattered on, which she doesn't really care for." Horrocks has been told that Edith Piaf used to urinate on stage. "So if I'm compared to her, I don't really mind." This is not, she insists, sensationalism for its own sake. "It is extreme, but there are extremities in the world. I've not seen Macbeth on stage before, but in Hamlet it really bugs me, that mad scene with Ophelia tripping round in long hair offering everyone bunches of cress. I've never seen a mad person behave like that - the mad drinkers who hang around the Tube station where I live are pretty scary. And yet people play those scenes like they want to be loved, like they want the audience to say 'Wasn't that moving? Oh poor little thing.' I prefer them to be done like in The Music Lovers with Glenda Jackson, when she was in that madhouse at the end, straddled over a grate and all these mad people were licking her," she laughs, almost shocked at the memory. "I found that intensely disturbing, but it stuck in my mind."
Horrocks has done some pretty visceral stuff herself in her time. She admits to having been "horrified" when she first saw her chocolate-covered sex scene as Nicola, the gruff-voiced bulimic in Mike Leigh's Life Is Sweet. "But it annoys me when people say, 'I've never met anybody like that.' You think 'Well, have you met everybody in the world?' I've met some very, very strange people." That particular character, it turns out, was based on someone she knew. Did the person concerned recognise herself? "I've never been quite sure... there were a few of her friends that knew."
Quite steely-eyed in her rejection of squeamishness - she insists, not wholly convincingly, but you know what she means, that "if it was a guy pissing onstage, no one would bat an eyelid" - Horrocks plainly gets a kick out of overturning stereotypically genteel notions of womanhood. "That's one of the reasons Absolutely Fabulous was so successful: because it let people see women in a different light; smoking dope, drunk and disorderly, and falling around in hedges. That's what people loved about it. They thought, 'Oh great - women being debauched: we've had men behaving badly, now here are some women doing it for a change.' "
For all her rejection of what she scornfully terms the "love me, love me" school of acting, Horrocks is not immune to criticism. She is still stung by the unfavourable comparisons of her Sally Bowles in Sam Mendes's Cabaret with Liza Minnelli's in the film. "The whole thing was," she says, "that Liza had a great voice and the character didn't - she was a second- rate performer in a second-rate club... I could see people in the audience getting ready to tap their feet and then the look on their faces was 'What's happening? She sounds more like Sid Vicious than Liza Minnelli.' " And did she enjoy that? "I loved it."
It would be wrong to conclude that Jane Horrocks is down on showbiz. Her performing career began as a teenage Mike Yarwood, inviting her secondary- modern classmates to judge the one-woman New Faces contests she would stage at lunchtime. Her gran used to sing a bit, but the stage was not in the Horrocks blood: her dad was a sales rep for an electrical firm, her mum a ward aid in a local hospital. At school Jane was, in her own words "a bit of a swot, but not very bright". Then, at the impressionable age of 15, she saw Barbra Streisand in A Star Is Born and thought, "Oh, I want to do that."
Her Streisand fixation quickly reached obsessive levels. "I went to get my hair done in a curly perm like Barbra's," she remembers, tenderly. "I took the album cover and said, 'I want my hair like that.' They told me it wouldn't come out that way, because her hair was much longer than mine, but I said, 'No, it'll definitely look like hers.' When they'd finished I looked about 90, the curls were lovely and tight. I left the salon with the album cover over my head."
Her exit from drama school a few years later was rather more glorious. Jane Horrocks walked straight out of Rada into a plum berth at the RSC. She must have been deeply hated by her peer group. "I don't think Ralph Fiennes and Imogen Stubbs were too happy. I think they were a bit surprised that of all the people in our year it was the girl from Rawtenstall [pron Rottenstall] who got into the RSC - I think they wondered why they'd been to Oxford. But then they both went there later on and did loads better than I did."
Horrocks's short time at the RSC was nowhere near as productive as the career she subsequently fashioned for herself. "There's a system there where you work your way up. When I left after a year people said 'What are you doing? You could be playing blah blah blah in years to come', and I thought I haven't got years to come. I don't want to waste my time thinking I may get Miranda in Twelfth Night eventually."
At 31, her keenness to chop and change shows no sign of abating: "You've got to keep trying different things because that's the only way to learn," she explains. "If you change with every job, people can't say 'Ooh, stick to what you know', because they don't know what you know." The only pigeon- hole anyone is going to fit Horrocks into is one with her name on it.
Macbeth - Greenwich Theatre, 0181-853 3800, till 28 Oct; then touring to Norwich, Malvern, Glasgow, Nottingham, Chichester and Richmond from 1 Nov (0171-434 9757 for information).