LOS ANGELES—In her new film, Hanna, Saoirse Ronan has hair so blond it appears white, and bleached eyebrows from beneath of which her eyes are a startling blue. She looks like an ethereally lovely forest sprite—the kind that snaps people’s necks like chopsticks and has exquisite aim with guns, knives, and anything else that’s handy. “I just missed your heart,” her Hanna says sweetly when she hasn’t quite managed to kill. Then she finishes things off.

In real life, in an L.A. hotel room, the 16-year-old Irish actor is explaining how to pronounce her first name. She is slender, her eyebrows are now visible, and she doesn’t look particularly dangerous. “Well, there’s a few different ways to pronounce it, actually,” she says thoughtfully. “ ”˜Seer-sha’ is how Irish people would pronounce it, usually. But I would pronounce it ”˜Sir-shuh’, like inertia.”

Inertia doesn’t exist in Hanna (which opens in Vancouver next Friday [April 8]). The titular heroine lives with her ex–CIA agent father, Erik (played by Eric Bana), in a cabin in a Finnish forest very near the Arctic Circle. When she’s not shooting elk with a bow and arrow and expertly removing steaming innards, she’s engaged in intense combat games with her father. Guns are frequently pointed.

“It was cold,” she says, recalling filming in Finland. “It was cold; it was cold; it was cold. One day it was minus 30 degrees. Fantastic. Eric and I, for the whole week or two that we were there, we were wrapped in a bit of deer fur and we had cloth around our hands, but they were fingerless. And we had to fight in this. We fought on a frozen lake on the third day. But it’s beautiful there, one of the most beautiful places that I’ve ever been to. It’s like a winter wonderland.”

Familial bliss is interrupted, however, when Hanna decides she’s ready to face the dangers of the outside world and embarks on a secretive mission for her father. Shortly thereafter she’s being hunted from Finland to Morocco to Germany by assassins led by an intelligence agent (Cate Blanchett) with a deceptively friendly Texas accent.

While preparing to kill or be killed, Ronan was no slouch herself. “I trained in martial arts and stick fighting and weapons,” she says. “I learned more skills than I’ve ever learned on a film. I don’t necessarily remember them all, but still.”

Before Hanna, Ronan hadn’t needed to learn the proper way to slit a person’s throat. Playing the devastatingly deceitful little sister in Atonement and the loquacious murdered teenager in The Lovely Bones, she won many awards. “I like characters that I’m thinking about for ages and ages after I’ve finished reading the script,” she says, “Usually ones that are different from me or something that I’m not or wish I was. Or maybe a piece of me that I haven’t brought out yet and I want to explore.”

She looks decidedly believable exploring her lethal side in Hanna. “Do I?” she asks. “Good. That’s good.”

Picking up a nearby butter knife, she bangs the end hard on the table.

“Ha! See, I’m getting the passion now to fight again. Better take this away from me.”