What are little girls made of? Sugar and spice, punches and the odd four-letter word, when they're the surprise star of Kick-Ass.

• Chloe Moretz is about to make quite an impact as Hit Girl in Kick-Ass. Picture: Complimentary

In real life, her bedtime is 9:30pm. Chloe Grace Moretz turned 13 last month and before she turns 14 she could become a hotter female property than her role model, Angelina Jolie. Later this year she stars in a remake of the cult vampire film Let The Right One In, and Martin Scorsese has just snapped her up to star in his next picture. But the reason we're sitting with Chloe, her mother, Teri, and two publicists – two more guardians, incidentally, than were deemed necessary when I interviewed Courtney Love – is Kick-Ass.

Directed by Matthew Vaughn, the film is based on Mark Millar's vibrant and hugely violent comic book series about a teenage loser who resolves to become a crimefighting superhero called Kick-Ass, despite lacking any real superpowers. The movie was supposed to offer a global launchpad for Nowhere Boy's British star Aaron Johnson, but the buzz is all about Moretz as Hit Girl, an 11-year-old who is half of a vigilante father-daughter team.

Moretz is not the star, she's not even the loopiest character in the film – not with Nicolas Cage playing her ever-loving dad as a combination of Pa Walton and Adam West – but Moretz is the character you will remember. In a purple wig, black mask, tartan skirt and fighting leathers, she screams at a denful of villains "OK, you c***s, let's see what you can do now" and proceeds to slice and dice them all with deadly blades and martial arts moves that would give Jet Li a nosebleed.

In our tiny hotel room, Moretz, dressed in Claire's Accessories pinks and blues, chats and smiles easily in between sucks on what appears to be an iced frappucino. So let's get the big question out the way first: did she expect her character's four-letter word bombs to have become such a talking point from the moment the movie trailers hit cinemas?

"It's a movie," she says, a little wearily. "Obviously a little girl can't beat up and kill huge heavy men. I don't see how anyone wouldn't realise that it's not real, it's Hit Girl. I'm just Chloe Moretz."

Has she ever used a four-letter word off-screen? "Never," she shrieks. "Ooof. My mom would ground me for, like, the rest of my life." Moretz can't even bring herself to say the film's title out loud in interviews, instead calling it "the film" and later she admits that at home they called the movie "Kick-Butt."

"I knew it was a controversial role, but it was a role I wanted to do," says Moretz earnestly. "A month or so before I got this script, Wanted (like Kick-Ass, based on a comic by Scotland's Millar] came out, and I really, really wanted to be a take-charge action heroine like Angelina Jolie." Who knows how Moretz became aware of an 18 certificate film about hipster assassins, but let's assume she saw TV trailers. "Anyway, when she read Kick-Ass, my mom was like, 'Chloe, you'll never believe this …' I read it and I was like, 'I have to be Hit Girl!' "

In the course of the movie, Hit Girl and her father fight crime with their fists, feet and the odd bazooka. Her most demanding stunts were doubled by an English gymnast, and a Chinese stuntman who specialised in gravity-grazing backflips, but Moretz is proud that some of the fights setpieces are Moretz herself, after four months of action training that included learning to load, clean and take apart a gun. "I learned martial arts moves, gymnastics, combat moves and knife handling," she counts off. "I actually learned to flip two balisongs (Filipino folding knives] at the same time, one in each hand too. And the last two months of it I spent at the Toronto Circus School! They took me there because they wanted me to get really flexible for the role."

Moretz has finally seen Kick-Ass herself, and pronounced herself relatively satisfied with her performance. "There are times when I'm running up to stab someone, and my face is going 'grrrrrr'," she says. "I was really happy with that, because the sequences are all choreographed and you have to start off on the right foot, or have your head a certain way, or the shot won't work. So when I'm looking like I want to kill someone, I'm really thinking, 'One, two, three, FOUR' in my head."

There's always debate as to the credit and debit sheet of being a child actor. They grow up mostly around adults and deal with adult-world paycheques and expectations: experiences that inevitably accelerate adulthood. And while Jodie Foster found there was life after Taxi Driver, she's outnumbered by child stars who end up working as taxi drivers. More immediately, there's the emotional turmoil of winning and losing roles. Moretz faced a rejection a couple of years back when she was abruptly dropped from the Disney animated feature Bolt, after recording the voice of Bolt the dog's preteen owner. Was she disappointed to be replaced by Miley Cyrus? Her bright smile wobbles only briefly.

"Well you get a thousand nos, and then you get one little shining star that says 'yes'. You just have to go with it. Miley is an amazing woman, she's very sweet," she says. Disney might be regretting the decision a little, since Cyrus is now hurtling towards adult roles and away from her old fanbase with almost indecent enthusiasm, while Moretz is currently hotter than steam and already making movies for both adult and teen markets. "Well, it's what they saw," says Moretz, evenly. "I'm cool with it."

Moretz got into acting when she was six. Her older brother Trevor trained at New York's Performing Arts High School, and his little sister used to watch him rehearse his lines, and later, helped feed him his cues. Her first major film was a remake of The Amityville Horror, and in The Eye she haunted Jessica Alba's dreams. Not all the roles in Moretz's CV are unsettling kids – there's Pooh's Super Sleuth Christmas Movie and Big Momma's House 2 – but it does seem that directors are attracted to Moretz's startling maturity as an actress. In (500) Days of Summer, she pulled off playing Joseph Gordon-Levitt's preternaturally wise little sister, and in the remake Let Me In, Matt Reeves's anticipated reworking of the Swedish modern classic Let the Right One In, she's thoughtful about her role as a 250-year-old child vampire in a way that leaves the kids of Twilight looking like dim bulbs.

"A lot of movies glamorise being a vampire but for my character it's a burden that she has to carry with her," she says. " It's like a devil inside of her, an alternate personality, and when it takes her over, she has no control. The director gave me a journal to write as my character, and I tended to think about little things: if you are 250 years old, could you remember even your parents, or the person who turned you into a vampire? I think it's these details that help you find a character."

With her knack for adult-oriented material, it's not surprising Moretz has to stress how normal she is. When she's not filming, Moretz attends school in Los Angeles, and insists her friends aren't interested when she disappears off to work with Jessica Alba or Donald Sutherland. They aren't even envious of the Tiffany starfish Nicolas Cage gave her when they wrapped Kick-Ass, chosen because he thought she was "a real star". But when her dad tried to open a can of fizzy soda for Nic Cage by stabbing it with his penknife, she sniggers like any 13-year-old at the memory of parent and actor getting soaked in Red Bull.

She's also marvellously unimpressed by Martin Scorsese, who is about to direct her in children's fantasy The Invention of Hugo Cabret. According to Moretz, Scorsese is a man who "talks really, really fast".

If Kick-Ass is a success, Moretz says she's keen to revisit Hit Girl ("Maybe with a purple Ducati to go with my hair") but she has already decided that in the long term she wants to use her acting superpowers for good and is a StarPower Ambassador for the Starlight Children's Foundation.

"Being an actress is so fun, but I also want to do something that will help the world. Helping kids in Africa and Asia," she says. "I want to be more than an actor who can just entertain."

Kick-Ass has its Scottish premiere at Cineworld, Glasgow, tomorrow night, as part of the Glasgow International Comedy Festival. It is released nationwide on Friday.