Something seems different about 19-year-old Saoirse Ronan from the moment the camera lands on her delicate, determined features in Kevin Mcdonald's youth-in-peril drama "How I Live Now," and it's hard to place exactly what it is. It's not the questioning Transatlantic accent, though that takes some getting used to. That piercing, pale-eyed gaze is one we know well by now, and the same goes for her quietly assured performing presence -- both present and correct in this unusual, genre-melding adaptation of Meg Rosoff's acclaimed teen novel.

 But as her character, Daisy, flicks her faintly punkish blonde bangs and tunes out everything but her headphones with typical teen contempt for the outside world, the penny drops: we’ve seen Ronan on screen throughout her adolescence, but at the same time, we’ve rarely seen her quite this, well, adolescent. “Preternatural” or “precocious” have long been go-to words for critics describing Ronan’s characterizations: the unwittingly powerful young meddler Briony in “Atonement,” her cool playing of whom earned her an Oscar nomination at the tender age of 13, the philosophical spirit of a young murder victim in “The Lovely Bones,” the socially unschooled kid assassin “Hanna,” or the possessed, two-minded alien of this year’s Stephenie Meyer adaptation “The Host.”

Like those characters, Daisy is required to face some very adult trials: set in the near-future, “How I Live Now” follows the snippy American high-schooler’s struggle for survival as the idyllic summer she shares with her English cousins is plunged into a devastating nuclear war, where the children are separated and forced to fend for themselves. But Daisy remains very much an ordinary girl in extraordinary circumstances – Ronan’s smart, flinty performance, which earned her a British Independent Film Award nomination for Best Actress last week, is colored by fast-switching teenage moods, from obtuse surliness to the undisguised blush of first love.

For Ronan, it was that very recognizability that drew her to the project. “I had begun to notice that I was playing a lot of characters who were quite otherworldly, whether supernatural or in some other sense,” she says in a cheery, ever-so-lightly muddled Irish brogue. “By the time I’d finished ‘The Host,’ I added it up and I’d played a vampire, an alien, a ghost, and a hit girl who had been hidden away in the mountains. I was desperate to play someone who’s part of modern society and pop culture. I felt it was something I really needed to do. And I loved the idea of playing someone from New York, with that personality and physicality and attitude – I was born in New York myself, so I could work a bit of my own background into her.”

Ronan admits to seeing herself in Daisy, but that wasn’t a particularly actressy impulse; most teenagers, she argues, would do the same. “I hope I’m not quite as stroppy as she is, but everyone has a bit of that going on,” she acknowledges. Certainly, Daisy isn’t an immediately likeable character, but Ronan doesn’t necessarily regard that as a challenge.

“I don’t really think of winning over an audience to a character – you just have to present her as she is,” she says. When I say that she seems to have been taking that approach ever since taking on the prickly young antagonist of “Atonement,” she agrees: “I was so young when I played Briony that I didn’t really think of how people might respond to her. But when you’re playing a character like that, or like Daisy, and you empathize with her, and understand what’s made her so cold or abrasive, then her vulnerability will begin to show. It’d be very boring just to have Daisy be a bitch the whole time, after all. It’s just like real life: some people you meet every day and they’re so harsh, but once you understand their circumstances, they open up. Characters require the same approach.”

Ronan hadn’t read Rosoff’s novel when she was approached for the role, though even before she received the script, the prospect of working with Kevin Macdonald, the tough-minded Scots director of “Touching the Void” and “The Last King of Scotland,” had her pretty much sold: “I was excited, because I love everything he’s done – and thought it would be really interesting to see what he’d bring to more of a teen romance.”

Attribute it to the source material, Macdonald’s direction, or most likely a bit of both, but “How I Live Now” indeed emerges as a very grown-up, rough-and-ready teen romance. That’s arguably part of a generational swing that has brought a harder genre sensibility to youth-focused storytelling, evident too in such teen heroines as Katniss Everdeen. “I hope that’s the case,” Ronan says, though she’s too polite to mention that her Hanna would probably have eaten Katniss for breakfast. “For so long, films about teenagers were so soft and formulaic, and there are more stories out there now with a bit of grit to them. ‘How I Live Now’ is obviously very different from ‘The Hunger Games,’ but I think there’s a similar vibe there, a kind of maturity.”

Guy Lodge is a South African-born critic and sometime screenwriter. In addition to his work at In Contention, he is a freelance contributor to Variety, Time Out, Empire and The Guardian. He lives well beyond his means in London.