The story of my lives

Lia Williams wanders in a daze along adual carriageway. She is playing Ella, the central character in May 33rd, a BBC1 drama that I have come to watch being filmed in the unlovely outskirts of Hemel Hempstead. The cheerless surroundings chime well with a Guy Hibbert-scripted TV drama as harrowing as you are likely to see this - or any other - year, and Williams, who appears in every scene, will confess in private to feeling exhausted by the role.

Lia Williams wanders in a daze along adual carriageway. She is playing Ella, the central character in May 33rd, a BBC1 drama that I have come to watch being filmed in the unlovely outskirts of Hemel Hempstead. The cheerless surroundings chime well with a Guy Hibbert-scripted TV drama as harrowing as you are likely to see this - or any other - year, and Williams, who appears in every scene, will confess in private to feeling exhausted by the role.

The 35-year-old Ella is on her way to visit an osteopath called Edward (played by the Danish actor Soren Byder). When he applies pressure to certain parts of her body, he is astonished to discover that it catapults her into one of five different personalities, each unaware of the others' existence. It emerges that Ella has been ritually abused since childhood by "The Family", a group of relatives and their friends. The trauma has caused her to suffer from dissociative identity disorder (DID), more usually known as multiple-personality disorder.

The actress is not unfamiliar with demanding roles: she has previously appeared in two tough dramas by Hibbert (her partner in real life): The Russian Bride (about mail-order brides) and Shot Through the Heart (about the Bosnian war). Williams also played the female student accusing David Suchet's campus professor of sexual harassment in the searing first UK production of David Mamet's Oleanna, at the Royal Court, London, in 1993. She is at home in Pinter, and on a lighter note will shortly be seen at the National opposite Charlotte Rampling in a play by Marivaux.

Nevertheless, the part of a DID sufferer is exacting, as Ella takes on her various personae - including a seven-year-old named Hannah and a tough teenager called Stevie - as a device to protect herself from the appalling memories of a lifetime of abuse. Ella inhabits a different realm from everyone else; as she puts it, "It's like living in a day that doesn't exist in anybody else's world, like it's May 33rd." Hannah is the persona Ella "escapes" into when she is haunted by a particularly upsetting memory (which can be brought on by something as apparently innocuous as a child eating an ice cream, a reminder of the treats she was given before and after episodes of abuse).

According to the actress, who visited a local primary school to study seven-year-olds' behaviour, "The childhood triggers that bring fond memories for most of us bring only nightmares for those born into an abuser network. Ella gives all the painful stuff that has happened to her in childhood to Hannah, so Ella can continue with her life. Hannah goes straight into the worst memories of abuse - being shut up in a tight space or begging not to be hurt. She is absolutely terrified and helpless. The strange thing is, Ella has no idea this is going on."

Being Hannah is so traumatic that Ella has also developed the "rescue" persona of Stevie. "She protects Hannah and gets her through the day," Williams explains. "She always comes out and deals with whatever situation Hannah finds herself in."

The actress goes on to give an example. "Ella is sitting in a café having a coffee at 10 o'clock in the morning. Suddenly, someone walks in wearing a T-shirt with a picture on it of Dougal from The Magic Roundabout, and that brings back a memory of childhood abuse and instantly triggers her into Hannah. During the time Ella is Hannah, she buys a teddy, gets on a train and ends up at the seaside.

"Then Stevie comes out and assesses the situation. She tries to work out whether a ticket has been bought [or whether Hannah has fare-dodged], what time has been missed while she's been Hannah, and how to get home. At nine o'clock in the evening, Ella reappears and can't understand why there's sand in her shoes. She has no idea what has happened to her day because the last thing she remembers is drinking a coffee at 10am. That sort of thing happens every day to people with DID."

Ella continues to need these personae because, it transpires, she is stuck in a cycle of abuse. "She compulsively goes back to being abused because, perversely, that's where she feels safe," Williams says. "When she tries to break away, her life is so shattered that society won't accept her."

DID is a tough nut for a dramatist to crack, as there is scepticism within the medical profession as to whether it exists at all - sufferers are sometimes dismissed as victims of "false memory syndrome". So, when I meet Hibbert - who has also written No Child of Mine (on the subject of child abuse), Saigon Baby (foreign adoption) and Bad Girl (single motherhood) - on set, I ask what drew him to the subject.

"After No Child of Mine went out, a woman who had DID as a result of ritual abuse contacted me. She said: 'If you write something about DID, people might start believing it.' When I met her, she triggered into six different personae in the course of an hour. It's very strange because you have to introduce yourself to each new person - and sometimes that person is a child. There is not a flicker of recognition that that person has met you before."

Hibbert continues: "What finally made me do it was this woman saying that her condition was caused by something so extreme - ritualistic abuse by her family - that no one believed her. There can be nothing worse than to have suffered that much and then to have other people accuse you of faking it. I'm in a privileged position in that I can tell a story to eight million people. I should use that privilege to help."

In addition, it has to be said, the story is a gift to a dramatist. "The basic premise is intriguing," agrees Hibbert, who conducted interviews with five DID sufferers and exhibits a compassion for them that prevents May 33rd from becoming voyeuristic or exploitative. "How do you get through the day if you're eight different people? How do you get from A to B? What if one of your personae decides to change the PIN code at the bank? When the next persona comes out, she can't understand why she can't get any money out of the cashpoint. As a writer, you think: 'That's a good starting point for a drama.'"

But how were the makers of May 33rd persuaded that DID is a genuine condition? "I don't find it that extraordinary," Williams declares. "Every day of our lives, we're changing our personalities. Now I'm talking to you in one way, and later I'll talk to my son in a different way. We're all role-playing all the time. For me, it is not incredible that women with DID have created this device to get them through the day. They may need a separate persona, say, to cross the road as they're frightened of cars because something horrific once happened to them in a car. It's a fantastic survival technique.

"But their lives are still torture - they are suffering in the most extraordinary way. The fact that they can switch into several personalities is the only thing that's keeping them alive. And it gets very complex because they aren't aware they are switching between personalities. I don't believe that level of complexity can be made up."

David Attwood, the director of May 33rd, has come to the same conclusion. "Observing a woman with DID, my initial reaction was: 'She's a fake.' But over four hours, I began to think: 'This is too well sustained to be fake.'"

For his part, Hibbert is not concerned that some viewers may find May 33rd too disturbing. "The point is to be as truthful as possible. It's like being a war correspondent who wants to tell you how appalling this war is, in the hope that someone back home will do something about it."

Williams shares the hope that May 33rd will raise awareness of a subject that for too long has been swept under the carpet. "By making people aware of DID, you're cracking open the taboo and saying it exists. You're de-stigmatising it. Since No Child of Mine, we can talk quite openly about child abuse. I hope that after May 33rd we'll be able to talk about DID."

So what's next for Hibbert? Having recently worked on dramas about the Omagh bombing, the sinking of the Russian submarine the Kursk, and the kidnapping of tourists in Rwanda, he is now embarking on a film about the civil war in Sierra Leone. Although he admits these subjects stay with him - "They so darken your mind that you feel the darkness will never be lifted" - he clearly can't kick his addiction to them.

Williams agrees: "Guy is brilliant at bringing a difficult subject into your home and making it tangible. He has a huge desire to expose injustice. However much he says he wants to do a light-hearted drama or a musical, he couldn't. Even if he tried to write a comedy, something awful would happen by the end of the second episode."

'May 33rd', 9pm, Wednesday, BBC1