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.”

That jives very much with Ronan”s personal taste: she”s never really been into what she terms “giddy” teen fare, and professes to avoid projects that she wouldn”t watch herself. Has anyone guided her to that sensibility, or has she always called the shots? “I”ve always been very lucky in that I have two great agents, one Irish and one American, who are pretty much on the same page as me,” she says, before handing equal credit to her parents. “They were very involved in reading scripts, so I had them for support and advice and opinions – but they never told me what I should and shouldn”t do. I simply gravitated personally to stories of children in a more adult environment.”

It”d have been easy to lose her bearings, as a number of prodigious child actors have done, after scoring that early Oscar nomination, but Ronan claims she never really felt the need to prove herself: “I just wanted to keep being involved with films that were that good,” she says. “I was 13, so I didn”t worry about my own career too much. Even now, as much as I want to make good films, you never know how it”s going to turn out. Once you”re finished with it, you”re finished with it.”

In fact, it took a few years for the young Ronan even to acknowledge that she had a career. “When I was a kid, even when I started making films, I didn”t even say I wanted to be an actress – I wanted to be a waitress.” She laughs merrily. “Honestly! I wanted to a be a waitress, a writer or an artist – but mainly a waitress.”

Ronan”s schedule is jam-packed these days. Nearest in the pipeline is a role in the all-star ensemble of Wes Anderson”s “The Grand Budapest” hotel, which premieres in February at the Berlin Film Festival: she jokes that Anderson will give her hell if she reveals too much about the character she plays, a bakery worker called Agatha, though the words “Courtesan of Chocolat” suggest we”ll be seeing her more playful side.

Is working on a Wes Anderson film as much fun as it looks? “It really is!” she enthuses. “You never forget that you”re in a Wes Anderson film – his style is very clear, very tangible in a way. There are hardly any visual effects, so when you walk on set, you walk right into his world: these beautiful art pieces have been made for you, and you”re in this amazing hotel lobby that has taken months to design and build. And Wes himself is like a character that he”s created.”

Ronan describes the experience of working with a cast that includes Ralph Fiennes, Jude Law, Tilda Swinton, Bill Murray and any number of other famous faces as “surreal” in the best way: “We”d all have dinner together every night, and it”d be like, there”s Harvey Keitel! And there”s Ed Norton! And Adrien Brody, and Tilda, and Bill, and all these amazing people. It just feels special to be a part of that.”

Most of Ronan”s scenes were shot with Fiennes and Tony Revolori, but she delighted in watching the rest of the puzzle fall into place. “I”d see bits and bobs of what everyone else was doing – and when I did, it was like watching another film I wasn”t a part of, which was really exciting,” she says. “Usually, I feel odd about watching footage on set, but when there”s so much that you don”t know anything about – like, you know, watching Tilda Swinton in a coffin or something – it”s a new experience.”

A different but equally rewarding experience, she says, was working with Ryan Gosling on his directorial debut, “How to Catch a Monster,” a dark, fantastical thriller in which she stars alongside Christina Hendricks and Eva Mendes. “I love Ryan,” she says. “As an actor, he”s one of the best, and as a director, the way he works is very interesting. A lot of it was improvised, so you”d turn up on set one day prepared for the scene, and then he”d say, ‘That scene you”ve just learned? We”re not doing that.” So you”d forget about the dialogue, and build something fresh.

“It”s really great, and I believe it”s quite similar to the work he”s done with directors like Derek Cianfrance. So he was very free with his approach, and had a lot of respect for our choices – but still had a very clear vision of what he wanted to achieve with the film. Plus, he”s just a great person to be around: very relaxed and self-confident, and when he”s your boss, it”s easy to be that way yourself.”

She speaks with genuine affection, though it hardly seems like Ronan needs to feed off anyone else”s self-confidence these days. That waitressing career may just have to go indefinitely on hold.