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Sienna Miller

Though she burst onto the Hollywood scene in 2004 as the beautiful ingénue in “Layer Cake” opposite Daniel Craig and, more famously, “Alfie,” with her ex Jude Law, Sienna Miller insists she’s really a character actor at heart.

Recalling how she desperately tried to talk “Lost City of Z” director James Gray into letting her sport the large, donut-shaped hairstyle favored by Edwardian-era women for the historical drama, Miller chuckles at the thespian instincts she’s developed over the course of her career. “Left to my own devices, I would have one eye and a hunchback,” she says.

In her natural state, which is still the fresh-faced, no-makeup makeup, tousled blonde hair and chic streetwear look, the 35-year-old actress grows more animated at the mention of her recent penchant for taking on richly detailed character performances, even if in secondary roles, in favor of high-glam star turns. She’s proud to have sought out more substantive and less ornamental parts.

“You do get slightly pigeonholed as a woman, and especially when I was younger,” she says. “I was quite flighty and frivolous, but really interested in character. People don’t want to give you that. You can’t sort of be both. If you’re a character actor, you have to be shy and weird, and I wasn’t. I was quite confident and ebullient. And now I’m shy and weird.” Playing a certain “kind of girl,” she found, was boring. “For me, the psychology of people is really what’s interesting, and the empathy that you have to find in, ‘What is it like to be that person?’”

In “Lost City of Z,” opening Friday, she plays Nina Fawcett, the spirited wife of real-life British-born Amazon explorer Percy Fawcett (played by Charlie Hunnam). The film’s sweeping tale allowed her to explore Fawcett’s life from her early 20s into her mid-60s.

“There was something beautiful about contemplating this woman’s almost whole life,” Miller says, having avoided aging tricks like an increasingly quavering voice or rickety movements in favor of subtler expressions of maturation, like casually but regularly rubbing her joints, “to kind of map out a span of a life, and try to find the moments where the experiences have affected her in some way.”

But her much-admired eye for style dovetailed well with an actor’s typical urge to build around props and costumes. “It really is for me where a character comes together,” Miller says. “I can do all sorts work on my own, but that costume fitting is really where it lands, hopefully.

“The clothes were extraordinary,” she says. “They were all original pieces, which is an incredible thing to put on. I’ve always loved vintage because it comes with a sense of history, There’s a sort of soul to these pieces. And because they’re original, they were really uncomfortable. The corset that I wore was an original whalebone corset.”

But Miller’s never been averse to a little suffering to achieve stylish splendor. “Every time I’m in a corset, I wonder why we don’t wear corsets in dresses. I love being held in that way, and the lace-up at the back is just so elegant. You carry yourself in such a beautiful way.”

Lately, however, she’s embracing a decidedly non-feminine aesthetic in her own style choices. “I don’t really know where it came from, but I’m quite into quite masculine workman’s clothes,” she smiles. “I keep getting drawn to big, baggy, sort of chimney sweeper trousers and tweeds. High-waisted, bunched up things and slightly odd shoes. I used to be quite flirty and sexy, and now it’s just like, ‘Do not fancy me’ clothes. The more unattractive I can make myself, the less flesh I can possibly show, the better.” (No matter that her vintage high-rise Levi’s and cropped sweater still look impossibly chic.)

She offers a conspiratorial aside: “It’s not technically true, because I do love a little frock now and then,” she says. She still admires designer labels including Gucci, Chanel, Calvin Klein and Sonia Rykiel. She also lists Trademark shoes; Mansur Gavriel bags; her sister Savannah Miller’s line, Nine, plus select vintage pieces as her favorites. And she carries an “endlessly” burning torch for Chloé: “I can hate it and see it’s got that label and be obsessed with it in a second,” she admits.

As much as she enjoys exploring other people’s lives on screen, she’s also hoping to carve out space to enjoy her own life, particularly with her four-year-old daughter Marlowe (whose father is ex Tom Sturridge). “The thing that I’m focused on is really trying to be as balanced as I can,” Miller says. “Having my work and my life, and both being clean and well-looked-after and nurtured so that both can be fulfilling — that’s a struggle. It’s really very hard. As I get older and I play these parts, I find the effect on me is more and more profound. It’s emotionally really draining, and increasingly so.”

She’s received notices for the harrowing parts she’s played in films like “American Sniper” and “Foxcatcher,” where, as the wife of men who met tragic ends, she’s had to plumb deep for inner grief.

“The key is to retain my safe little sanctuary of my life and my home, and disassociate from each when I’m there,” she explains. However, her life is about get more challenging: after a year away from filming, she’s taking on her first starring role since becoming a mother for the Ridley Scott-produced “The Burning Woman,” playing an immature, blue-collar woman whose life takes a tragic turn. A week after that shoot ends, she then heads to London’s West End to play Maggie in “Cat On a Hot Tin Roof.”

“I feel like I’m at the bottom of an Everest,” she confides. “I’m a little bit anxious, because they’re also polar-opposite women. I’ll just be a schizophrenic mess.” But she’s still ebullient at the core. “Once the play is up and running, I have the days with my daughter. You pop to the theater in the evenings and it’s summer in London, and it will be lovely.”