Sienna Miller recounts 9-hour sex scene with Ben Affleck

'There will be some outtakes from that [scene] where I have to walk out of the room, because I just have tears running down my face'

Even for the most committed and professional of actors, there's no quite getting around how utterly ridiculous filming a sex scene is.

Having to get half-naked and flop around on top of your co-worker while 20 grips stand around checking their watch and wondering when lunch is - it's a weird, weird aspect of what's already an admittedly surreal job. If there's a way to laugh through it, then inevitable any actor is going to be pretty thankful for that opportunity.

Sienna Miller was pretty thankful for Ben Affleck, certainly; taking on the role of Irish moll Emma Gould in Live by Night, both directed and starring Affleck as gangster Joe Coughlin.

"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! OK. How do we do this?'" Miller told E! News. "Obviously, by the time nine hours of it has gone past, I was shaking with tears running down my face. I mean, I can't tell you—but you have to laugh. Ben is just professional. It is what it is."

"It's hysterical," she added. "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. In that environment, it was a cool scene."

At one point, Affleck apparently instructed cinematographer Robert Richardson to "keep rolling" so they could film the same sex scene "a few times" without having to cut. " I was like 'OK, obviously Joe Coughlin is a real performer.' This happened three times, and by the third I [finally said], 'Are you joking?'" she joked. "And then I was like, 'I don't know what you're trying to tell people, Ben, about your sexual prowess—your abilities.'"

"It turns out that he was just giving himself options without cutting," she added. "But to me it was the funniest thing. I was like, 'No one can do what you're doing, the marathon love scene. There will be some outtakes from that [scene] where I have to walk out of the room, because I just have tears running down my face."

Affleck himself jokingly responded to Miller's story with: "we were starting over, like all scenes. You start the scene—you get to the end—you go back to the beginning. Sienna thought that I was trying to make myself out to be some kind of world champion, where it just never ended. I just keep going, going, and going again and again. She started laughing and I was like, 'Laughing isn't really the response that I'm looking for with this.' She was like, 'How long can you go? You're Superman!'"

"The camera was still rolling. I didn't tell her ahead of time, so she was taken a little off guard. I thought she'd just roll with it."

Live By Night hits UK cinemas 13 January 2017.