Sienna Miller has said she realised a childhood dream by appearing as a gangster’s moll in latest film Live By Night.

Set in the prohibition era in Boston, the movie sees Sienna, the mistress of an Irish mob boss, engage in a passionate affair with Joe, played by Ben Affleck, who has just returned from combat in the First World War.

Speaking at the film’s European premiere at the BFI Southbank on Wednesday night, Sienna said she had enjoyed dressing up as a character from 1920s America.

Sienna Miller
Sienna Miller (Vianney Le Caer/AP/PA Images)

She told the Press Association: “To play a gangster’s moll was a dream.

“Every fancy dress party from when I was 10 I would dress up as a flapper so to actually get to go to work and do it, it was bliss.”

The British-American actress made headlines earlier on Wednesday in the US after she discussed the nine hours of sex scenes she and Ben endured during filming.

She joked that Ben, who also directed the movie, was trying to boost his reputation for sexual stamina.

Ben told the Press Association: “Sienna accused me of trying to make myself look like a much more impressive lover than I really am in the sex scenes.

“I was trying to do multiple takes, it wasn’t that I was actually trying to pretend that in real life I could do multiple takes.”

The Oscar-winning actor praised Sienna as a “magical actress”.

Ben Affleck
Ben Affleck (Ian West/PA Wire/PA Images)

He said: “She’s great, she’s fabulous. I had a first rate cast.

“She’s a prodigy. I don’t think there’s anything she can’t do.”

He also said he had his “fingers crossed” for brother Casey Affleck who was nominated for a Bafta for his role in Manchester By The Sea on Tuesday after winning a Golden Globe earlier this week.

Ben said: “I’m thrilled, I’m hoping he finds success here in London. Londoners are known for their great taste so I hope Casey does well.”

Live By Night opens in cinemas on January 13 in the UK.