Gillian Anderson plays an enigmatic detective in BBC's suspenceful new mini-series The Fall

On the wall of the Zetter hotel in Clerkenwell is a rnonochrorne painting of a pale, raven-haired woman. She is wearing a corseted ballgown that becomes, from the waist down, an elaboratebird cage.Staring darkly out at the viewer, she holds a dagger aloft.

I tell Gillian Anderson - who sits in front of the painting, slight blonde, in black jeans and a blue silk shirt - that the woman strikes rne as exactly the kind of tormented part she would play. 'You're absolutely right,' she says, beaming.
The gulf between Anderson, 44. and the characters she portrays on-screen verges on comic. She is celebrated for playing troubled, earnest women: po-faced and sceptical Dana Scully (The X-Files), haughty Lady Dedlock (Bleak House), ghostly Miss Havisham (Great Expectations) and now the elusive detective Stella Gibson in the BBC's new drama The Fall. But in person she laughs, frequently, and finds comedy even in the ordering of coffee. She asks, very precisely, for a decaf Americano with cold whole milk, and when the jug of semi-skimmed arrives, skips to the bar to plead with a waiter to milk a cow and bring her the proceeds.

The role of Stella will inevitably draw comparisons with Scully, the part that fixed Anderson in the hearts of sci-F i devotees the world over. After a brace of costurne dramas, she is back in the present, investigating a case - not aliens, but a serial killer (creepily played by ex-rnndel Jamie Dornan) in Northern Ireland. Watching Anderson in hard-jawed-detective mode feels comfortingly familiar, but for her it's new territory: 'They're so different. Scully was so square.' She points out that you'd never see Scully stripped to her underwear and straddling a policeman, as Stella is in episode one. This is a portrait of a modern, complex woman - whereas Scully, she says. was quaintly old-fashioned, Stella doesn't have to play second fiddle to a partner.

Anderson scuttles her fingers across the table, miming perhaps the most infuriating aspect of her career on The X-Files: Scully permanently two steps behind Mulder, the leading man.

Yet Scully is hard to shed. If you've played an iconic part early on, it can stalk you for years. At the time, the only way Anderson could deal with the attention was to distance herself from the world: 'If I paused long enough to think about it, the reality of it would have been too great to bear.' She recalls seeing an advert for the box set with her face on it: 'Part of my brain was going, "I know that person." But it was a remote version of herself, and it was only after the series ended. when 'I was someone else... that I was able to go. "Wow, that was really cool and I was a part of that. Good for me."'

Anderson flew from fame, returning to England, where she had lived as a child. She is a passionate Londoner, proud of her British-flavoured humour and in possession of a precise English accent. 'Every day I feel grateful that I live in this city... walking down any street and hearing 10 different languages as people pass you. Amazing.' The city's architecture and history stun her, too - she looks out of the window across a rain-soaked Clerkenwell Road, points out a plaque above a doorway as though it's the most beautiful thing she's ever seen, and swings back to tell me about the Chinese boats that gathered on the Thames hundreds of years ago. 'It was extraordinary!'
Anderson's electric enthusiasms have their fipside: she is obsessively, exhaustingly busy. Life is planned months ahead, her days a frenzy of emails and appointments, most of which, she says, feel oddly pointless. She is the 'complete opposite' of the cool, controlled Stella. 'I would aspire to be as content in herself as she is. What I do know is that my busy-ness is hiding something... for whatever reason, I have a really hard time letting myself relax.'

It's not connected to her return to single life (Anderson recently separated from her partner, businessman Mark Griffith, the father of her two sons Oscar, six, and Felix, four). 'I think it's just how I am... I feel like I'm just getting to a point where I've had it about up to here,' she says.

An ideal life, in her mind, is one with more time; time she would spend creating her own work, looking after her children (she also has an older daughter, Piper, with her first husband, Clyde Klotz), and working for charitable causes beyond the whorl of fundraisers, which are 'not enough of a sacrifice'. Her dedication to good works - for organisations such as the Neurofibromatosis Association and Artists for a New South Africa - is already formidable, but Anderson, it soon becomes clear, does everything full-throttle. Despairing, for example, of her inability to retain information, she hired a tutor. ('I'm very passionate about things and I have a lot to say, but I can't necessarily back it up and that terrifies me.') They started at the Russian Revolution and worked through the 20th century, filling in the gaps left by high school, when she was 'on another planet'. But her old rebellious ways returned: 'I didn't do my homework.'

I like the sound of Anderson's teenage self - lost years living in Grand Rapids, Michigan. She was a tearaway and thrift-store junkie, clad in ripped tights and buckled boots, dresses three sizes too big pinned to her body with belts ('It was that grungy, gothy, punky thing'). Nowadays her approach to fashion is similar, if a little more upmarket. The day before the Baftas a couple of years ago, she met William Banks-Blaney, owner of William Vintage in Marylebone. She had nothing to wear, stopped by his shop and fell in love. 'I put on a couple of dresses and I literally felt like I'd never known what it was like to wear a proper dress before... I've been awakened to the world of couture, or the world of borrowing couture.' Still, she insists, she has no idea how to prettify herself, and reduced the crew on The Fall to hysterics when she tried to do her hair in a scene and burnt her neck with the hairdryer. 'I'd even practised in the trailer!'

This is another of Anderson's beguiling British traits: the fine art of self-mockery. She might not be controlled, or even content, but she's a breezy world away from the dagger-wielding woman painted on the wall.

The Fall will be shown on BBC two in the spring.