Sienna Miller has revealed she made her “young-looking” boyfriend kiss her in public while wearing prosthetic make-up that aged her, just to confuse onlookers.

The actress said that people were “horrified” at the sight of her 28-year-old partner Lucas Zwirner “kissing his nana”.

Miller, 37, played the wife of the late Fox News founder Roger Ailes – who was forced to resign from the network amid a sexual harassment scandal – in the miniseries The Loudest Voice.

Lucas Zwirner and Sienna Miller
Lucas Zwirner and Sienna Miller at Ellie Goulding’s wedding in August (Peter Byrne/PA)

The role of Elizabeth Ailes meant the British film star was required to wear special make-up and prosthetics to transform into the older woman, something she used to her advantage while out with American writer Zwirner.

Miller told The Graham Norton Show: “I used to take ‘her’ for walks and into bars.

“I walked down the street with my quite young-looking boyfriend and asked him to kiss me passionately in front of a crowd of tourists.

“They were horrified at the sight of this young boy kissing his nana!”

The Loudest Voice
Russell Crowe and Sienna Miller in The Loudest Voice (Showtime)

The Loudest Voice, which airs on Sky Atlantic in the UK, stars Russell Crowe as Ailes and the cast also includes Seth MacFarlane and Naomi Watts.

Miller also said that, for her most recent role in the film American Woman, she only realised during filming that it was the first time she had a lead role without a male actor by her side.

She said: “It occurred to me as I was filming that I had never been on my own in a lead role before.

“I had an amazing cast supporting me but for the first time I didn’t have a man to lean on.”

In the film, Miller plays a working-class single mother who must put her life back together after her daughter goes missing.

Graham Norton Show – London
Host Graham Norton with (seated left to right) Bruce Springsteen, Robert De Niro, Sienna Miller, Paul Rudd and James Blunt during the filming for The Graham Norton Show (Isabel Infantes/PA)

The film is set over a period of 11 years and, after premiering at the Toronto International Film Festival, she has received positive acclaim from critics.

Miller appears on The Graham Norton Show alongside Robert De Niro, Bruce Springsteen, Paul Rudd and James Blunt.

The Graham Norton Show is on BBC One at 10.35pm on Friday October 11.