Keira and Sienna frolic on the sand in preparation for steamy session

Taking time out between takes of romantic drama The Best Time of Our Lives, the Hollywood stars frolicked along the Wales beachside and giggled like schoolgirls.

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The stars are teaming up with actor Matthew Rhys for a steamy threesome scene - Rhys plays Welsh poet Dylan Thomas, who has two women competing for his love.

Miller, who plays Thomas' wife, stepped into the role after troubled actress Lindsay Lohan pulled out over monetary negotiations.

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Knightley has spoken of her real life affection and admiration for her co-star.

She recently told Elle magazine: "I wish I was Sienna Miller."

"At the parties, I stand on my own, grinning aimlessly. I can never think of a thing to say. I watch that girl and she's just fabulous. She talks to everyone and laughs and smiles and looks as if she is genuinely enjoying herself.

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"When I talk to her I hope a bit of her party personality will rub off on me, but it never does. She could give master classes in being the most amazing party guest."

For Knightley, the film is also a family affair -it was written by her playwright mother Sharman McDonald.

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Miller is proving to be a silver screen ménage a trois specialist of sorts.

She has taken up the role of threesome-loving 1960s figure Louise Ferrier in the film Hippie Hippie Shake, which starts shooting in September.