“I had a dream last night,” says Emily Watson. “I dreamt that I was about to go on stage and I didn't know the lines and I didn't even know what play I was supposed to be doing... I was running round the theatre surreptitiously looking for a copy of the play to find out exactly what we were doing.
“And every time I walked down a backstage corridor, it turned into a forest, and I woke up at two o'clock and thought 'Phew!... thank God that was just a dream.' And then I fell asleep and went straight back into the same dream. It's an actor's dream we have all the time, but it taps into a basic fear that I am going to be very exposed.”
This impromptu excursion into Watson's nightmares was prompted by our discussion about her latest role. She plays Dr Yvonne Carmichael, an eminent scientist and a “pathologically law-abiding” wife and mother, who finds herself in the dock of the Old Bailey, in Bafta-winning Amanda Coe's BBC adaptation of Louise Doughty's best-selling novel Apple Tree Yard.
Carmichael is a geneticist and, when we meet her in Coe's adaptation, she is heading to the Houses of Parliament for a routine appearance before a government committee, helping it to pronounce on recommended limitations on cloning technology. “It's the same debate we've been having for years,” she says, secretly scornful of the limitations of MPs' knowledge of her specialist subject.
What this highly self-controlled woman doesn't herself know is that before the morning is out she will be enjoying hot, animal sex with a mysterious stranger in the Chapel of St Mary Undercroft, beneath the Palace of Westminster – an act so uncharacteristic that Carmichael returns home and stares incredulously at her reflection in the mirror.
The lover whose name she doesn't even inquire about until later (in her late-night diary, written while her husband sleeps downstairs, she refers to him as 'X', as in the X chromosome “or the most innocent of kisses”) is called Mark Costley. He is a man with an opaque occupation (a civil servant? A spy?) – played with suitably vulpine charm by Ben Chaplin. Either way, this compulsive dalliance will lead Carmichael to the dock in a murder trial – the less revealed about which here, the better.
“It's particularly important that she was a scientist, because she's spent her whole life being rational,” Doughty tells me. “And now through one rash act, her life is spinning out of control.
But back to Watson's nightmare, and her “trepidation about being that exposed”, because not for the first time in her career, the role requires what television continuity announcers are apt to call 'scenes of a sexual nature'.
“When I first read the script I thought 'really?' It made me very nervous,” says the actress, who turned 50 this month. “But then I went to meet Jess [director Jessica Hobbs] and she completely turned me around.
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“I feel that something like this has come at a time in my career when the roles for women of my age in movies and television are smaller and smaller and smaller, and this is about a fully grown-up woman who is powerfully sexual and has a complex life. Those women aren't really represented on screen.”
Even so, Watson was clear about certain boundaries. “The first thing I said to Jess was that I wouldn't do any nudity,” she says. “It's not that I'm a prude or vain it's just that I'm way too old to be naked on screen.” And what might have been a demanding shoot was made far easier by the casting of her old friend Ben Chaplin (The Wipers Times, Snowden) as Costley.
“One thing I felt Ben has done with the part of Costley is that he has taken away any sense of it being sleazy,” says Watson. “So I hope the audience will fall for him in the same way Yvonne does. So when things turn, it becomes as much as a shock to them as it is to her.
“When you're asked to do those kinds of scenes when you're 22 there's just a lot of fumbling and hoping and just wishing they'd say 'cut'. But when you've got two actors who've been round the block a bit, we just sat down and discussed it point by point and had a plan, because we wanted it to seem real, and like a progression. I've known Ben for a long time and we were very honest about it... straightforward... and it was fun.”
Watson mentions being 22, but it was at the age of 28 that the London-born actress, then a virtual unknown outside the National Theatre and the Royal Shakespeare Company, stepped into the lead role – after Helena Bonham Carter "got cold feet" – in one of the most controversial films of the 1990s, Lars von Trier's Breaking the Waves. The proto-Dogme 95 movie would win Watson an Oscar nomination for her very first film role.
She played a young woman in a remote Scottish community who submits to sexual degradation out of love for her paralysed husband and because she believes in God's will – a role that required nudity and graphic sex scenes.
“I wasn't comfortable but I did it because I wanted to give myself completely to this role,” she says. “I was very naïve – I'd never made a film so I didn't know why this was different from other things, but evidently it was. It just completely put me on the map. I was an actress in demand after that.”
Despite a lot of what she calls “come hither... come to Hollywood and earn a lot of money,” Watson sought out interesting roles (opposite Daniel Day-Lewis in The Boxer, as cellist Jacqueline du Pré in Hilary and Jackie, and in Robert Altman's Gosford Park), while turning down Amélie (which had specifically been written with Watson in mind). “I don't speak French,” she says. “And I just had an instinctive feeling that I would make a tit of myself.”
Then there was the Olivier Award for her stage performance in Uncle Vanya at the Donmar, and a Bafta for Appropriate Adult, in which she played an inexperienced charity worker employed to sit in with serial killer Fred West (played by Dominic West) during his police interviews, while The Politician's Husband in 2013, opposite David Tennant, showed that she was still up to sexually explicit parts.
“These roles just come my way really... I don't seek them out,” she says. “Once you done it once people think 'Oh maybe she'll do it again'. And I think you have to be a bit of an idiot really – just jump all four feet in and regret it later and have that ability to forget.”
All the same, Watson believes that there is a political point here. “I think there's a lot of debate going on about the presence of women in film and television and pieces like this are becoming more common,” she says. “The most loyal audience for TV is women and they are really interested in watching women like these on TV. The cultural topic of sexually powerful women in their fifties is now open for business.”
'Apple Tree Yard' begins at 9pm on 22 January on BBC One
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