Keira Knightley's favourite sex scene was in 'Atonement'.

The 33-year-old actress has done her fair share of stripping and acting out racy bedroom scenes during her career, but she has admitted the ''best'' X-rated moment was when she joined her co-star James McAvoy - who portrayed her character Cecilia Tallis' lover Robbie Turner in the 2007 drama classic - for intimate scenes on top of a bookshelf.

Speaking to Vulture, she said: ''The best sex scene I've done on-screen is the one in 'Atonement', on the bookshelf.

''It was both the best sex scene, but also [the best] to shoot.''

Keira - who was tasked with performing lesbian sex scenes in her recent film 'Colette' and also gets intimate in her latest period drama 'The Aftermath' - said that it was the director Joe Wright who made her 'Atonement' sex scene feel ''comfortable'' and not at all ''exposed'', through his precise choreography.

Explaining that he ''choreographed the scene within an inch of its life'', she added: ''It was absolutely, 'Foot goes up there, hand goes up there. So both me and James [McAvoy] felt utterly comfortable and not exposed, and like we could deal with it. It's never gonna be fun, but we could deal with it.''

Keira also got 'The Aftermath' helmer James Kent to take control of her steamy scenes with Alexander Skarsgard the same as Joe.

She said: ''What never helps is when a director goes, 'Oh, you guys know what you're doing.'

[And you're like], 'Uh, I've never met this person, I have no idea what I'm doing in this room full of men. ''With sex scenes in particular, it always has to be choreographed like a dance.

''So with James [Kent], I was like, 'You're the director, you know what you want from this, so you have to just tell us. And everyone will feel comfortable.'''

And James learned a lot from Keira's advice, and will now always approach sex scenes in the same way.

He added: ''Keira asked me to narrate - [she said] otherwise, you're too conscious that we're doing it.

''But if you narrate, it takes the awkwardness out of it, it becomes technical.'''