The role of Nina ages from being a young mother to a woman in her 60s
If there's one thing Sienna Miller has in common with the character she plays in exploration epic Lost City of Z, it's that she's not afraid to air her views, loud and clear.
So much so that she altered a script when she thought Nina Fawcett was appearing as too much of a stereotypical "wife" - with the director framing the scrawled-on piece of paper.
The film - based on a true story - is about explorer Percy Fawcett, played by Charlie Hunnam, and his quest to find a mysterious lost city in the heart of the Amazon.
His first mission in 1906, accompanied by surveying partner Henry Costin (played by Robert Pattinson), sees them return with apparent evidence that this previously undiscovered community exists.
Percy is drawn to return again and again, while his wife supports him and raises their family.
'Brave and stoic'
But one of Nina's first lines sees her complain about how restrictive a corset is - and her inability to be contained and constrained by society is a running theme.
"In the script, I was adamant that she wasn't just a wife - because she's a suffragette and she's a mother, and she was incredibly brave, self-sacrificing and stoic," says Miller.
"She was a huge part of the engine that ran this entire discovery; she found the piece of paper that proved there was a Z. She was just tireless in her support and her love.
"I can't imagine what it would have been like to be a woman then, and I wanted to represent the frustrations of how that would have felt, and not ask for pity, which I think is an easy trap to fall into when you're playing someone who's raising children and is left behind.
"She was very defiant and I loved that about her."
Charlie HunnamImage copyrightSTUDIOCANAL
Charlie Hunnam said the character of Nina was a "badass"
The star of High Rise says Nina "wasn't your typical early 20th Century housewife", and that she and Percy - who were both Buddhists - were "forward-thinking, progressive people".
She says: "What film can do is give you a sense of what something would have felt like and the empathy that comes from that, and acknowledging how far we've come and how much further we have to go.
"I hope it's thought-provoking, and I hope it's emotive. That's all you can hope for when you make a piece of work."
Miller says she doesn't think the film would have portrayed Nina as anything other than "just a wife" if it had been made even just 10 years ago.
"On paper, initially I did read it as sort of a wife part," she says. "Both James [Gray, the director] and I were aware of that and worked really, really hard to ensure that wasn't the case.
"We were doing a script read-through and I was sitting next to James and there was a page that still felt very 'wifey'. I wrote in the middle of the read-through, in black letters, 'This. Needs. Work.' - like that, while people were reading, and underlined it.
"He's framed that piece of paper now.
"I was just pioneering for her and he was as well. He's a feminist man. Ten years ago, or in the hands of the wrong director, she would have been a complete afterthought.
"But he's a good man with daughters and a great wife and he understands that women should be appreciated and heard."
Sienna MillerImage copyrightREUTERS
Miller says she "loved" playing an older woman in the film
Gray says of Miller's role: "Sienna's an actress who brings great resources emotionally to the role of Nina. It was always refreshing to be on set and watch her do things you knew she was capable of but hadn't yet shown to the world."
And Hunnam says Nina was "fearless and sassy", adding: "Percy was obviously a real badass of the time, but so was Nina."
Nina ages from 20 to 60 in the film - and Miller says she is grateful to the prosthetics department.
"For me, it was just heaven, because you've an arc and you've something to map," she says. "What's this experience done to her? Is she brittled? There are so many knocks in her life, and I just wanted her to hold on to that hope.
"You can represent a life, basically, so it was really interesting. Left alone in the prop cupboard, I would have one eye and a hunchback, so I'm all up for prosthetics. And, it was interesting to play this older lady. I loved it."
One of Miller's next roles is Maggie in Cat on a Hot Tin Roof at London's Young Vic, opposite Jack O'Connell.
She's previously taken on West End roles in As You Like It at Wyndham's and Flare Path, but said of the production of the Tennessee Williams play: "It's amazing, but hugely demanding - with a lot of dialogue."
But, she adds with a smile: "I'm so excited about it - slash absolutely terrified."
Lost City of Z is out in UK cinemas on 24 April.