Then a 38-year-old mother of five and trainee social worker in Gloucestershire, Leach was called out of the blue one evening in February 1994 while making her children's tea, and asked to attend the police station immediately, as an individual who had volunteered to act as a so-called "appropriate adult". Appointed to sit in on police interviews with West, it was her role to safeguard the interests of a suspect deemed to have learning difficulties – but she had no idea what she was about to witness. West's interrogation was the first time she had acted in the role, and she was not only party to months of harrowing interviews, but also was obliged to escort West to 25 Cromwell Street, where many of his victims were buried.
The case was the subject of uncomfortably close media interest. It had come to the nation's attention after the disappearance of West's daughter Heather prompted police to excavate the garden at his house in Gloucester on 24 February 1994. The next day, West was arrested on suspicion of Heather's murder – but it was evident that hers was not the only body buried behind 25 Cromwell Street. West was subsequently charged with 12 murders – but, in the run-up to his trial, the 53-year-old committed suicide on New Year's Day 1995 in Winson Green Prison, Birmingham. His wife, Rose, was arrested in April 1994. She was convicted of 10 murders in November 1995 and is now serving a life sentence in Low Newton Prison, Durham.
In the period between his arrest and his suicide, during dozens of encounters, West had come to trust Leach. More remarkably still, he had told her about many further killings which he had not previously confessed to the police but with which he was eventually charged.
Sitting in an office overlooking the Thames, Emily Watson recalls her own first meeting with the woman she is about to portray. "I knew a lot about Janet and was very curious about her," she says. The producers of Appropriate Adult had interviewed Leach many times in the past four years, and in preparing for the role Watson read her responses. "She seemed incredibly vulnerable and damaged by the experience; I expected her to be timid and downtrodden. But when I met her, there was something steely and strong about her, which I really liked. You don't survive something like that without getting really tough."
Written by Neil McKay, the drama has clearly been meticulously researched – Leach has seen the film and is supportive of it, and Gloucestershire Constabulary has advised on the piece since 2007. But that hasn't stopped the protests, including a complaint from Anne Marie Davis, Fred West's daughter. Her mother, Rena Costello, and half-sisters Charmaine and Heather, were all murdered by the Wests and Davis believes the film "will cause unimaginable distress to the families of the young girls who were murdered".
Watson understands this anger, but defends the producers' right to make the drama. "It is tricky because the subject is very painful for people," admits the actress, "but I don't think it's in any way disrespectful. It is handled very sensitively, and the producers were very scrupulous. Every single line had to be signed off by the legal department. The process was absolutely painstaking – rightly so with a subject like this. But of course there will still be people who are unhappy about it. If it had happened to someone in your family, would you want Fred West's name mentioned in your house?
Watson adds that she anticipated a media furore ever since she took on the role. "I was completely expecting it because people haven't seen it yet and they don't know the nature of the drama. They assume and want it to be lurid so they can be outraged.
"But I maintain that it is not lurid, so hopefully they won't be outraged. It's an intelligent attempt to tackle this subject. There are no reconstructions of the crimes or body parts. It's not salacious – it's based on transcripts of interviews. It's all about two people talking in a room. I hope it will give people a greater understanding of our stupefaction that someone could be living in our midst for 20 years doing those heinous things and still be seen as everyone's mate."
While they are undeniably compelling, the exchanges between West and Leach make for chilling subject matter. There are echoes of The Silence of the Lambs in the way Leach is mentally seduced by the magnetic, yet deeply manipulative West. And through these conversations, it's possible to glimpse the roguish charm he used to lure his victims.
Watson has always been an actress known both for the intensity she brings to her work and the impeccable choices she makes: she has awards for work in such diverse films as Lars von Trier's Breaking the Waves and the Jacqueline du Pré biopic Hilary and Jackie (both of which earned her Oscar nominations), not to mention Gosford Park, Angela's Ashes, Red Dragon and Synecdoche, New York.
She delivers another acting masterclass in Appropriate Adult. In several scenes she has no lines, but she doesn't need them. In the first interview she sits in on, for instance, Leach listens to West casually relay how he strangled Heather and disposed of her body; the horror is written all over Watson's dumbly uncomprehending face.
For all that, the 44-year-old confesses that she hesitated before accepting the role. "I did have reservations about it, absolutely," she recalls. "When I heard about it, I said, 'I don't think so.' Then I read it and changed my mind. It's a really thoughtful attempt to do what drama should be doing – exploring the nature of humanity. This is a pretty extreme corner of that.
"Also, this is a relationship we have never seen before on television, between a serial killer and an appropriate adult. In most dramas, you play someone falling in love or getting upset about their child. It is very unusual to portray someone like this."
In fact, Leach emerges as something of a hero. "She was a vulnerable woman who obeyed the rules," says Watson. "Unbeknown to her, that was the key to opening the case. Suddenly West felt he could unburden himself to this woman. He trusted her and became dependent on her. He told her things he didn't tell anyone else. She made him confess. Without her, we might have heard about only one or two murders." And many victims' families would never have discovered what became of their relatives.
Such was the rapport between West and Leach that when she was unavailable, he refused to speak in the presence of her replacement. When he confided in Leach that he had committed nine more murders than he had previously admitted, she was bound by confidentiality and so was unable to relay this information to the police. However, by then West was so dependent on her that he panicked when she threatened to resign as his appropriate adult. She said she would remain in the job only if he confessed everything to the police, which he duly did.
It was at this point that Leach's bond with West progressed into a greyer area. "Janet continued her relationship with him and visited him in prison, which is where it becomes questionable," says Watson. "But her only motive was to get the truth. Janet believes to this day that there are more victims [of West] we still don't know about."
An expert emotional manipulator, West frequently exploited Leach for his own ends. According to Watson, "She was seduced by him. Not in a romantic sense of falling for him, but she was taken in. He was brilliant at manipulating situations and changed his story all the time. Also, Janet found herself at the centre of a national story, and that was intoxicating. There were times when she stepped over the boundary, and her involvement with him became too personal." Leach maintained contact with West after he was jailed and awaiting trial; she even offered to help him find a new solicitor. In the end, as Appropriate Adult reveals, her contact with West was to have a devastating effect on her health.
Is there a risk that a drama starring as charismatic an actor as Dominic West might make us sympathise, however fleetingly, with West? "It does initially veer towards that," admits Watson, "but as the scales fall from Janet's eyes, so they fall from the audience's eyes. We're supposed to be sucked in by West, but as Janet suddenly sees how badly abused she has been, you realise that it has all been total manipulation. We don't know how we would respond in that situation. None of us would be equipped for that. Janet acts as a proxy for us all.
"Everyone involved in this case entered a moral universe for which they were completely unequipped. They went into a chasm full of utterly depraved things. The usual rules did not apply," adds the actress, who admits that immersing herself in this world has taken its toll on her, too. "You don't go into a drama like this without coming away with its smell on your clothes," she says. "It lived with me. I came away traumatised. I was glad when it was finished. It was a very intense six weeks. It was a highly engaging acting work-out and I love that. But it was not a good place to dwell too long."
Where better for Watson to turn than the rather more wholesome embrace of Steven Spielberg? The actress will next appear in early 2012 in the director's big-screen version of War Horse, an adaptation of the hit West End play, itself based on the book by Michael Morpurgo. Watson will play the hero Albert's mother, and says: "I loved the stage play and signed up for the film before reading the script. When Steven Spielberg rings you and asks, 'Will you come and meet me?' you instantly say, 'Yes!'"
For the time being, Watson is happy to return to her day-to-day life at her London home with her husband, the actor Jack Waters, whom she met at the RSC, and her two young children. "If you did this job full-time, you'd be dead," she says. "It's so exhausting. You can't be at that intensity all the time. You don't have any kind of life. Sometimes you need to pause in order to shed one skin and grow another."
'Appropriate Adult' starts tonight at 9pm on ITV1
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