Meet EastEnders Kat's alter ego

She doesn't move like Kat. Jessie Wallace's walk is more cautious. Wary. She isn't wearing heels, so the signature killer strut - hips forward, back arched, chin stabbing the air in front of her - isn't in evidence.

She doesn't dress like Kat, either. Not really. Her jeans are tight, but jeans, as opposed to too-clingy pencil skirts or micro-minis. Her top is muted, a sleeveless crewneck with rosette ribboning on one shoulder. Her coat is chunky and unrevealing. But she laughs like Kat - loudly, suddenly, often. Her estuary trash accent is Kat's. Her hair - which Wallace dyes dolly black herself, with home kits - is Kat's. So is her St Tropez-coated skin. It's not entirely surprising that mix-ups happen.

Jessie Wallace doesn't like the mix-ups. As her public becomes increasingly besotted by EastEnders' Kat Slater (Wallace's fictional hell-raising alter ego), and as the character becomes more triumphant and avenging, more cartoonishly lascivious and shouty, so the press becomes increasingly desperate to blur the division between soap and reality. Everyone wants Kat to be real.

Three days before we meet, Jessie makes the front page of The Sun following a public bust-up with fellow cast member Hannah (daughter of Dennis) Waterman at a Michael Parkinson after-show party. The report claimed she'd hit Waterman, after Waterman had tried to stop her verbally attacking an unknown partygoer. This came relatively hard on the heels of a story claiming that Jessie had walked into a gay bar, done up to the nines, and shouted: 'Is there anyone in this bar who isn't f***ing gay?' And another one, which maintained that Wallace had hit a minicab driver following an argument about a fare, called him Osama Bin Laden, and stormed off. Where does Kat end and Wallace begin? From this distance, it's impossible to tell.

Today, Wallace is promoting a week-long, high-octane EastEnders courtroom extravaganza. Her soap sister, Little Mo, is on trial for the attempted murder of her abusive ex-husband, Trevor. Viewing figures are expected to hit 18 million. I, meantime, am here to determine whether or not Kat Slater and Jessie Wallace are the same person.

I expect a diva. Partly because Kat is purest, unadulterated diva, partly because Wallace has become so astonishingly famous so quickly. But what I get is a sweet, semi-evolved celebrity. Admittedly, Wallace is a diva-ish hour-and-a-half late ('I love my lie-ins,' she says, before realising she's been caught out schedule-busting, and blustering: 'Oh, and the traffic was terrible!') but apart from that, she's phenomenally ordinary. She loves having her hair done, trying on clothes, cooing over jewellery. She's not yet jaded by the publicity process - she clearly still finds it all glamorous, exciting, luxurious. 'They treat you so well on these shoots, don't they? Trying to relax you.'

She's also furnished with an unpolished line in interview clich?. Wallace insists, for example, that she didn't ever mean to be famous. 'Not famous, no. To act, yeah. But not to be famous.' It's all quite endearing.

She was an attention-seeking child, she admits. 'But you often hear actors saying that, don't you?' (She bases this observation on the information she's picked up from obsessive viewing of Sky's biography channel, on which the lives of Marlon Brando etc are dramatised.)

She grew up in Epping - her parents split when she was three, and she and her older sister, Joanne, were brought up by their father, James. Her mum lives in Berkshire. She was constantly cast as the lead in school plays - Cinderella, Dorothy - and always intended to be an actress, but got distracted along the way. She became a barmaid, a market trader, a wig maker.

Wallace landed the EastEnders role a little over two years ago, after giving up on a career as a theatrical make-up artist ('It was brilliant. I did prosthetics and everything') to re-train as an actress at the North London drama college, The Poor School. She had an epiphany, standing in the wings, watching the first night of Simon Callow's Les Enfants du Paradis, which she'd worked on. She realised that that was what she wanted to do. 'It was hard, to go back to studying and struggling again. But I wanted to do it. It was a struggle. Such a struggle. You just keep trying. It got me down loads of times. Like when you can't even afford toilet paper. I'd have these times at drama school, when I was pretending to be a lizard crawling round the floor, thinking: "All my mates are in the pub now. I'm a lizard. What am I doing?"' She wound up cleaning the school to pay her fees. 'I'm proud of that, actually, That's how strongly I wanted to be an actor.'

It took EastEnders three months to cast Wallace as the character now credited with rescuing it from a plot slump and revitalising its ratings. When her agent told her she'd got the part, one entire, jobless year after she'd graduated, she blacked out. 'When I woke up, my face was all wet from crying.'

Has it delivered everything she expected?

'No,' she says. 'Much, much more.'

The Kat Slater effect is staggeringly potent. Kat's - or, rather, Jessie's - image can now shift more magazines and newspapers than Posh Spice's. (Wallace doesn't know this. When I tell her, she's staggered. 'You are kidding. You are kidding. I did not know that. I sell more magazines than Victoria Beckham. Really? Let's go and tell her. I wish I had her house.') Kat's storylines have a genuine impact on people's lives. When the character revealed that she'd been abused by her uncle when she was 13, one woman felt able to bring charges against a man who'd raped her years earlier. Wallace's scenes were played in the court as evidence.

In short, Kat is an inspiration, a national treasure, a card-carrying icon - a status more usually reserved for the non-fictitious. We want to believe she's real, that Kat and Jessie are interchangeable. Accordingly, Wallace has developed an uneasy relationship with her fame. The first time she confronted it, head on, she was terrified. 'I was walking through Soho. And all I can say is, it's like walking around with no clothes on. I was scared. I was really scared.' Now, mostly, she finds the public's reaction funny. 'I was in this club in the toilets putting lipstick on,' she remembers. 'And this girl was looking at me in the mirror, and she went: "You think you're that bird, don'tcha? You think you're that bird off the telly?" And I thought, "I don't know how to answer that."'

The press reaction, however, is a different matter. Wallace and I skirt around the issue of the recent tabloid stories nervously, but eventually address it.

Does she ever get depressed, I wonder.

'I do, yeah,' she says. 'I like things to be fair. If I've done something wrong, I'll hold my hands up to it. But when lies are being said... that really gets me down.'

Like the Hannah Waterman story?

'Yeah. Because that did not happen. We had a disagreement, and that was as far as it went. But unfortunately, there were journalists there.' Wallace lapses into damage-limitation speak, practised sound bites. 'And they're only doing their job. But they blew it out of proportion. Completely out of the water. There was no fists that flew, there was no slapping.'

So the two of you are friends?

'We're friends, yeah. But there was a disagreement. I do a little thing, and it's blown out of proportion, because people want me to be like Kat. That's a prime example. You have a heated discussion, voices are raised, but they do a thing in the paper and, to make it interesting, they have to add.' It was the same, apparently, with the gay bar story, and the Osama minicab story.

I ask if she's developing strategies to deal with it. 'At the moment, I feel like I haven't got a hard shell. This stuff's really got to me. There's that saying, "Today's news is tomorrow's chip paper", but I'm sick of hearing it. It's not, anyway. They use normal paper.'

There's little about Wallace's attitude to suggest she shares Kat's rabid sexual appetite. She's currently single. She split from her long-term boyfriend, Menorca-dwelling millionaire Paul Whitworth, earlier in the year. She insists that she's happy being on her own, and working. She fancies people, she admits. But she won't say who.

She lives in a house in Epping, with a cat called Elvis and a dog called Bailey, and the collection of chandeliers that she can't help adding to compulsively, but can't find sufficient space to hang. She's extravagant, she admits. 'Yeah, but so much so that's it's not good. It's not good. And I saw this one chandelier in this shop and I bought it, and it's not even on the wall because it's fit for the Royal Albert Hall. It's just sitting on the floor. The cat sits on it. I'm the worst saver in the world. I've got big holes in my pockets. For years, I had to go without, so now, if I see something I want...'

Wallace's entourage arrives in the shape of Nicky, her oldest friend. Nicky works in retail, for the Arcadia Group, and has just flown into town following a three-day break in Spain. She and Wallace have matching tans. When the two interact, or reminisce about their past, Wallace becomes undeniably Kat-like. She speeds up, laughs more. 'We met in the Salisbury, in Bushell Park,' says Wallace. 'We were drinking. In the youth club. Wasn't it, Nick?' Nicky nods. 'When I was 16, I used to make jewellery and sell it on a market stall. I used to get Nicky to try all the things out. I made these earrings out of Fimo and sprayed them with the paint you use on cars. Nicky went out with them in the rain, and they stuck to her face and all the paint ran. I didn't make those again.'

I ask Nicky how she'd describe Jessie. 'Funny,' she says. 'Always making me laugh. Like the time she cut her hand.' Wallace flashes a scar on the flap of skin between her thumb and forefinger. 'I was rolling out the Fimo using this pint glass for a rolling pin, and it broke in my hand. It was so deep you could see the bone. My sister was eight months pregnant at the time, and she fainted.' She should have gone to casualty, but she'd bought tickets for a big club night, and was determined she wasn't going to miss it. She bandaged up the wound and carried on, regardless. 'It got infected and it smelled like a ham sandwich. Remember, Nick?'

I ask her how it feels to be a gay icon, but Wallace isn't convinced she qualifies. 'Mo is, but I don't know about me.' She did G.A.Y. this New Year's Eve, however, introduced Lulu and started the countdown. 'I was so nervous that I did it backwards. I went, "20, 21, 22," and they were going "19, 18..." I went, "Oh yeah, 18, 17..."'

There are, clearly, differences between Jessie Wallace and Kat Slater. Wallace is less vulnerable, less hyper, lower-key, gentler. Kat's more brittle, more complex, harder, uncompromising. And, of course, not actually real. But there are also gratifying similarities. Wallace admits that Kat's spirit takes her over occasionally. She has Kat moments. Almost like Kat Tourette's.

As for Wallace - and accordingly, Slater's - future, she doesn't have any long-term plans. She's blissfully happy in Albert Square, though she thinks she might like to do theatre eventually. She hasn't ever practised her Oscar acceptance speech. It hasn't even crossed her mind.

Could she ever imagine herself retiring?

'No. There's so many different things I want to do. I'd like to open up a little shop somewhere, renovating furniture.' She stops. Smiles Kat's deliberate, lewd smile. 'Or I was thinking of bringing out a brand of cheese called Kat's Crack.' The moment the words are out, she slaps a hand over her mouth and looks appalled. 'Oh my God! I can't believe I said that! You can't put that! It's too rude!' Like she says. She has her Kat moments.