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Live by Night's Sienna Miller and Ben Affleck snuggle close on the red carpet

Michelle Johnson

They spent hours perfecting steamy scenes for their new film Live by Night, and the ease and friendly affection between them made sure Ben Affleck and Sienna Miller had the perfect red carpet partnership at their London premiere on Wednesday evening.

The 35-year-old actress looked beautiful in a black-and-nude Lanvin slip dress despite the cold. But dapper Batman star Ben was heroically on hand when the wind picked up, wrapping her in his overcoat as they spoke to press on the red carpet.

And Sienna, who plays the femme fatale who captures Ben's character's heart in the film, had nothing but praise for her director and co-star on the red carpet.

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"To work alongside Ben, with his track record, I love the films he's directed and as an actor, so it was all great," she said. "I think he's obviously incredibly intelligent."

"Starting from Good Will Hunting, which he wrote when he was so young, he's just really got what it takes," she added. "And he's a lovely human being, so it was a lot of fun going to work on that set. It was just a very fun to go."

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Ben Affleck and Sienna Miller star in Live by Night

The fun extended into Live by Night's steamy love scenes – which took more than nine hours to film – with Sienna admitting to E! that the pair got through with laughter.

"Ben and I are like brother and sister, thank God, so there was no awkwardness. There was just a lot of stupid giggling. He's very professional – I am not, but he is," she said.

"In that environment it was a cool scene. There was a montage in the script saying we did it everywhere: in the car, in the bar... I was like: 'That's an entire day of just love scenes! Okay. How do we do this?'"

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Ben was heroically on hand on the red carpet

The mobster film, directed by Oscar-winning Ben, is set during prohibition-era Boston. Ben stars as Joe Coughlin, son of a policeman who turns to crime after his wartime experiences, who soon falls in love with glamorous gangster's moll Emma (Sienna) the mistress of Irish gang boss Albert White (Robert Glenister).

"I love gangster movies," said Ben. "I think they're the quintessential stories of ambitious and our fantasies of living outside the law."

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"I think he's obviously incredibly intelligent," said Sienna of her co-star

"I wanted to make a movie like they use to make: the classic gangsters; the cars; the guns; the fedoras; the girls," he added.

"It's an old-school, rip-roaring gangster film," said Sienna. "I love those film and hope everyone else does as well!"

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