Keira Knightley lights up the London Film Festival in a cross-my-heart dress at A Dangerous Method screening

By Emily Sheridan

Last updated at 12:18 AM on 25th October 2011

Keira Knightley's latest film A Dangerous Method sees the actress back in conservative period clothing for the umpteenth time.

But as she attended a London Film Festival screening tonight, there was nothing old-fashioned about her sexy dress.

The 26-year-old showed off just the right amount of flesh for an October evening in her black and grey Roksanda Ilincic dress with a cross-my-heart wrap top detail.

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Slender: Keira Knightley shows off her slim frame in a black and grey Roksanda Ilincic dress at A Dangerous Method screening in London

Form-fitting: The dress hugged the actress's delicate curves in all the right places

She teamed the ankle-length dress with satin peep-toe heels and a swept back updo as she posed on the red carpet at the Leicester Square screening.

Knightley was joined at the festival by the film's director David Cronenburg and her co-stars Viggo Mortensen and Michael Fassbender.

A Dangerous Method tells the story of the relationship between psychoanalysts Carl Jung (Fassbender) and Sigmund Freud (Mortensen).

We love you! Keira borrowed a friend's jacket as she went round signing autographs for fans

Knightley plays Russian psychoanalyst Sabina Spielrein, who has an affair with Jung and ends up with some saucy spanking scenes.

In a recent interview, the actress admitted downing a vodka before filming the spanking scenes.

She said: 'I did a couple of shots of vodka - definitely - beforehand, and then a couple of glasses of champagne as a celebration of never having to do that again.'

Glamour girl: The combination of the dress and heels made Knightley look even taller than 5ft 7in

Knightley admitted the spanking initially put her off the script and she phoned Cronenberg to decline the offer of the part.

She explained: 'When I first read the script, I thought, the script was fascinating, and it's David, and I really want to work with David, but I read those two scenes and just went: "I don't think that I can do that," particularly because it's the age of the internet; it's gonna be everywhere.

'I phoned him up to really turn it down. I said to him, "Look, I love you, I love the script, I love the character, but I really don't know that I can play those scenes."

Leading men: Knightley was joined at the screening by co-stars Michael Fassbender (left) and Viggo Mortensen (right)

What's so funny? Knightley shares a joke with director David Cronenberg

'He said, "Look, if I'm gonna do them, then they're gonna be clinical; they're not gonna be sexy, they're not gonna be voyeuristic in that way.

'And I thought, "Okay, well I can understand that. As long as it's clinical and it's not some, sort of, weird sexy spanking thing."'

A Dangerous Method is released in the US on November 23 and in the UK in February 2012.