When the long-running alien conspiracy drama The X Files ended in 2002 after nine years, its flame-haired star Gillian Anderson was at a professional crossroads. Her success on American TV, as the cool, adroit FBI special agent Dana Scully, made her a household name. Greater things should have followed. In the eyes of casting directors, however, she was a square peg in a city of round holes.

"It is a particular dilemma," Anderson says. "With someone like Jennifer Aniston, who was on television for a long time playing a specific character, what she has to offer in Friends is easily translatable to film, in terms of romantic comedy. You can see the marketable options.

"With Scully, I think, producers and directors have a hard time seeing how having Scully in their films would be beneficial and marketable. What they're not doing is taking into consideration that I might actually be able to do something other than Scully, so when someone is given my name they think, 'How does Scully fit into it? How does that seriousness or that aspect she brings to the screen fit in?' "

Faced with a choice between repetition or reinvention, she chose the latter, moving to London, where she had spent a decade as a child, and taking the first steps towards a transformation that is confirmed in the BBC's latest adaptation of Charles Dickens's Bleak House.

It is an outstanding adaptation from the pen of Andrew Davies (Pride and Prejudice, House of Cards, Tipping the Velvet), and Anderson, in a sweeping, powerful performance, easily silences her doubters. Boy, can she do more than Scully!

Bleak House is a $20 million spectacle with an all-star cast: Denis Lawson, Nathaniel Parker, Timothy West, Charles Dance and Anderson, with Liza Tarbuck, Roberta Taylor and Catherine Tate in smaller roles. Like the novel, originally published in monthly chapters between 1852 and 1853, it is a complex, costumed soap about a long-running legal dispute, Jarndyce and Jarndyce, and those embroiled in it. In the series, Anderson plays Lady Dedlock, the proud and vain spouse of the idle, fashionable Sir Leicester Dedlock (West). Lady Dedlock's terrible secret - which we will not reveal here - is one of the main plot points and an explosive twist to an already intriguing story.

Anderson has never felt comfortable with the kind of attention that success on TV inevitably brings. When I first met her, in 1994, on the set of The X Files in Vancouver, Canada, she was shy and cautious. In Sydney for a promotional tour in 1996, she was visibly uncomfortable with the intrusion. When we spoke then, she equated fame with "suffocation".

Today, far more comfortable as a serious working actress living in London's fashionable inner city than she was as an employee in the Hollywood sausage factory, she seems at ease with the attention. "I think the reason I ended up here - other than the fact that I knew in some capacity I always would, having grown up here and feeling a strong connection to the city - was needing to get as far away from the world I had been living in before," she says.

She was drawn to Bleak House, she says, for three reasons: an outstanding script, a professional production and the fact it was such a dramatic departure from the kind of character she was so closely associated with on American TV.

"The mixture of those three things and the fact that I had been told ad nauseam by friends who were both actors and watchers of the BBC that a costume drama is very, very different from an American television series - with all those elements in place it seemed like a really good decision."

Lady Dedlock is a wonderfully complicated character. She is Bleak House's tragic protagonist, a charming, proud and steel-jawed aristocrat, whose concealment of secrets ultimately threatens to be her undoing.

"I seem to be drawn to women who have secrets and she certainly has her fair share of secrets," Anderson says. "I found there was a lot to work with as an actor. She was complicated and at once cold and harsh and stern, yet pleasant and warm, and you can see her potential for love and innocence. I really liked her."

At the heart of the character, Anderson says, is her fear of exposure. "People were much less forgiving then than they are today, of people who fall out of social graces, and her fear of being found out certainly sculpts the way she behaves in the world. The stakes are quite high, and as a result she has to be quite severe and withholding, and that cannot help but create a cold, and withholding personality."

Before accepting the project, Anderson read the novel - though she is quick to point out that such research isn't always beneficial, as it risks encouraging actors to play something that's not on the scripted page. "I just did a film called The Last King of Scotland, based on Giles Foden's book, and the script was different enough from the book that I think it would have been a disservice for me to have read the book beforehand, because I would have questioned choices the writer made. With Bleak House, I was reading the book with Lady Dedlock in mind, so I probably didn't pay as much attention to the idiosyncracies and plot points of the other characters as I might have if I weren't."

Anderson is full of praise for Davies's adaptation of the sprawling, 1088-page novel. "Dickens goes on and on for pages about particular scenes, characters and personalities, and Andrew's job is to make it dramatic enough for television ... I think he did a brilliant job siphoning out the leanest, juiciest aspect of the characters and the drama and the important aspects of the text."

The success of the project speaks for itself. Radio Times critic David Butcher praised the series and Anderson in particular, whom he said was "at the heart of the drama. It's a magnetic performance in a tremendous piece of television."

Since Bleak House, Anderson has finished work on two films, The Last King of Scotland and Straightheads, and is soon to start work on another, No One Gets Off in This Town.

Reinvention, though risky, seems to have paid off. "It's a matter of me doing enough that is completely different [from Scully] to show them that there is a larger range and different possibilities," she says. "At the same time, I have chosen to be here. I'm not back there pounding the pavement or showing up to the meetings, and in terms of American film, I don't at this moment really mean much.

"But I am not willing to sacrifice my integrity in order to put myself in a position where I might make myself more bankable or marketable. It sucks, sometimes, because there are films that I see on the screen that I think would have been great to be part of, and yet I am also getting to live my life. I love living here and I love doing what I'm doing."