The French actress and her co-star had to romp around in a cramped car for their characters' racy encounter and Cotillard reveals they rehearsed the scene to make sure they had the "choreography" correct.
"We had a very determined choreography," she tells WENN. "When you know what your body can do it allows you the freedom because you won't think, 'Oh, what am I gonna do next? Am I going to go on top of him...?' It's an awkward situation.
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"So with (director) Bob (Zemeckis), we sat in this fake car with two seats and we laughed because you need to get it out because it's so weird. And then we were like, "We're gonna do this, and then I'm gonna go on top of you and then we're gonna kiss and then blah, blah, blah'. When it's decided it's just easy."
Zemeckis admits he's glad he decided to rehearse the scene in advance and he didn't just ask his leading man and lady to go for it.
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"It's so crucial that we choreographed that scene ahead of time," he explains. "It's exactly like doing a dance number. It's all just movement and rhythm. It all just becomes a dance."
And Marion is thrilled with the finished scene, telling Extra, "It's one of the highlights of the love story."
In the film, which hits cinemas on 23 November, 2016 the actress plays a French resistance fighter while Brad is an intelligence officer.
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