INTELLIGENT, vulnerable, neurotic, in love with, but slightly contemptuous of, her husband, certainly not evil and a few other contradictory adjectives that I hope I incorporated.”

Olivia Williams is talking about Ruth Lang, wife of the British prime minister in The Ghost, the film of the Robert Harris bestseller about the intrigue surrounding the writing of the PM’s memoirs.

As the author is a former political advisor and friend of a certain Mr Tony Blair, a man not unknown in the corridors of No 10, many have taken the book to be a thinly-veiled commentary about the former leader.

Which means Williams is Cherie Blair.

Or not. Confusion is inevitable although the actress had clear ideas about playing Ruth. She certainly didn’t want to do an impersonation, not least because she believes Helen McCrory did a brilliant job as Cherie in Stephen Frears’ film, The Queen.

“I’ve been out in LA for the past five months and nobody out there knows or gives a damn about them. If this film is going to stand up as a political thriller worldwide it has to be based on the characters being interesting in their own right,” she says.

“There’s nothing weirder than watching an impersonation of someone you don’t know who’s being impersonated.

So, we just had to start from scratch and look at what Robert Harris gave us, which was incredibly rich not only from the script but the novel. I started scouring the text and harrassing Robert with emails about how he wanted her to be.”

As a result, he sent her that list of adjectives mentioned in the opening paragraph.

Harris was certainly describing Cherie Blair in the book (“dark hair, Northern working-class political roots”). “You knew it was her but didn’t want to impersonate her, so I took each one of those adjectives and passed them through the conduit of my body and face and that’s what I came up with,” she explains.

“Cherie is brilliant source material because her predicament, as is Hillary Clinton’s predicament, is of an intelligent woman but there’s something about the way she says things that puts people’s backs up. And she never got over that and that’s Ruth Lang’s problem as well.

“It’s a public perception and often worse for women than men – they don’t like these intelligent abrasive women.

The bizarre exception to the rule is Margaret Thatcher, and how did she do it?

She got under the wire because she did the extraordinary elocution thing of lowering her voice. She sounded like a man, dressed like a hunting, shooting upper middle-class woman and managed to make people think she wasn’t from this aggressive female intellectual tradition.”

Williams is that rare thing, an actress who doesn’t toe the bland publicity line while promoting movies, but engages in interesting, and even controversial, conversation.

Since Kevin Costner chose her as his leading lady in The Postman, she’s produced a varied body of work on film, TV and in the theatre. She recently spent two years in the US in the TV series Dollhouse, from Buffy creator Joss Weldon.

Playing a character called Adele “dressed up to the nines, lovely lighting and lovely designer outfits”, she recalls “A lot of Adele was about presentation.

How she was perceived was very important to her, whereas Ruth in The Ghost doesn’t give a toss how she’s perceived.

The only time she puts on a bit of lipstick is when she’s trying to get information out of the ghost writer.”

This past year she’s been seen in two movies, An Education and the Ian Dury biopic Sex & Drugs & Rock & Roll, as well as Dollhouse. “An amazing year and now I need to put my feet up. I have two kids,”

she says. “I love working. The theory is I’m going to take a break but I love my job”

She has no idea what Cherie Blair thinks about her portrayal, although has an idea of her husband’s reaction.

“Tony’s apparently just very chuffed he’s being played by Pierce Brosnan,” she says.

■ The Ghost Writer (15) opens in cinemas on Friday.