Meet the stars of Scorsese’s new blockbuster

In a New York hotel suite, two 14-year-olds are having a teenage argy-bargy.

"Hi mate, yeah, oi, I'm a British boy..." says glamorously confident Chloë Moretz, affecting the throaty tones of a moody adolescent Cockney oik.

"I don't speak like that," tuts well-spoken, wide-eyed, fidgety Asa Butterfield. "You use our language so don't even..." The Londoner stops and glares at the Hollywooder sitting next to him. When I ask him if his voice has recently broken, she titters. "I think it's in the process of breaking. This morning it was going ooe-e-aah-ooh," says Butterfield in a strangulated wail. The young Brit has also recently taken to Twitter to boast that he's now taller than the American.

"Barely," Moretz says witheringly.

"She might be taller than me right now," says the five-foot-sixer ("and a half"), looking at her elaborately shod feet, "but if she took her heels off she's like a little midget."

"I am five-five and still growing," she pouts proudly.

But right now you're Little Midget Moretz...

"I AM NOT MIDGET MORETZ!" she shrieks in the manner of someone who's been called this before. "I am Very Tall, Amazing Moretz."

Roll over Diaz and tell DiCaprio the news: Martin Scorsese has a new pair of leads, and they're brilliant squabbling teens. Butterfield and Moretz are the hero and heroine of Hugo, the director's first kids' movie. An adaptation of Brian Selznick's bestselling 2007 novel The Invention of Hugo Cabret, the 3D film tells the story of an orphan (Butterfield) living in the secret passageways of a Parisian train station in 1931.

His late, horologist father has bequeathed Hugo a love of clockwork and a broken mechanical man. The boy's endeavours to fix the automaton are aided by tools and trinkets he steals from the station toy booth, run by an old man (Sir Ben Kingsley) known to his goddaughter Isabelle (Moretz) as Papa Georges. Hugo, who is constantly on the run from the officious, Great War-damaged station inspector (Sacha Baron Cohen), becomes friends with Isabelle – and they embark on an investigation into Papa Georges' mysterious past, and the very origins of cinema.

Scorsese has conjured an entertaining, wondrous children's adventure. Almost as entertaining is watching his young stars – clearly good friends, they spent nine months together in London's Shepperton Studios filming Hugo – bicker and score points. Moretz has the bigger international film career: she was the sweary, all-action Hit-Girl in the comic-book film Kick-Ass (2010), and the vampire in Let Me In (2010). Butterfield is best known for The Boy in the Striped Pyjamas (2008). Accordingly, she's beating the pants off him in the Twitter follower stakes. As of this morning's hotel meeting, she has 134,318 followers. He has 5,525. "Pretty good though," Moretz offers encouragingly, "seeing as he started Tweeting, like, at the end of Hugo."

Moretz – all extrovert and fashiony – has been acting solidly since she was six, her family relocating from Atlanta, first to New York then Los Angeles, in pursuit of her dreams.

Islingon-born Butterfield – quiet, fashiony only under photoshoot duress – has had a more gentle trajectory, and still attends a comprehensive in Hackney. She is, in many ways, miles ahead of him.

And you can tell. According to Moretz's Twitter feed, she's had a giddy few weeks of Katy Perry concerts, Teen Vogue covers ("A Star is Born") and swanky dinners with Chanel in honour of Pedro Almodovar. According to Butterfield's, he's been suffering from double-maths homework and a week of "not talking to anyone" while he and his cousin played the newly released computer game Battlefield 3.

"Wait!" chips in Moretz on hearing this news. "I beat Call of Duty: Modern Warfare 3 in one day."

"That's because," opines Scorsese's new leading man, "Call of Duty: Modern Warfare 3 is for pussies."

"Shhh!" gasps the great auteur's latest leading lady. "You can't say that in interviews. Oh my God, you're so not PC."

Scorsese makes his first children's film – and following Robert de Niro, Leonardo DiCaprio, Sharon Stone and Cameron Diaz, you're his lead actors. How does that feel?

Moretz: "Special. It's a huge honour. We're still young, and a lot of people don't get to work with him ever. And I was the only American in the movie. So that was scary too."

Butterfield: "Pretty epic. Cos he's such a renowned director. But yeah, basically what Chloë said."

Describe the audition process for Hugo.

Moretz: "We heard that Martin Scorsese was making a kids' movie and he'd like to see me for it. I did my whole audition tape in a British accent. So the whole time he thought I was actually British, until we flew out to New York to do the 'chemistry' read together. At the end he was like, 'Ah, you're American!' 'I am! Good thing I fooled you!' And they called us on a weekend to tell us, which is huge. They never do that."

Butterfield: "I got pulled out of school by my mum and she told me I'd got it. Then I went and had a carrot juice."

What's Sacha Baron Cohen like?

Butterfield: "You expect him to be all cracking jokes, 'Ees nice', Borat-ish. But he's very serious. And he completely stays in the role. Even when the camera wasn't rolling, everyone would call him station inspector. He even had the calliper on his leg, hobbling around."

Are Hugo and Isabelle the embodiment of the young Scorsese, enraptured by movies and dreams and possibilities and escape?

Butterfield: "I think so. Chloë is the energetic, mystical side of the story. I'm the more mischievous, adventurous..."

Moretz: "...spontaneous piece of Marty. I definitely think we are Marty split into halves. This movie is a love-letter to cinema from Marty. And I think he put a part of his heart into it, especially for his daughter Francesca – she's 12 and it's one of his only movies she can actually see at her age."

Butterfield and Moretz suddenly realise that today is Scorsese's 69th birthday.

The only present the actor rustled up for his mentor was a card from the hotel gift shop. "But last year I did get him a Nerf gun." The actress baulks as, first, she realises she's bought him nothing and second, "My God, we've spent two birthdays with him!" After completing the long shoot on Hugo, Moretz stayed on in London to film – alongside Johnny Depp and Helena Bonham Carter – Dark Shadows, Tim Burton's upcoming fantasy epic.

Butterfield's first on-screen appearance was in a 2006 Boxing Day TV drama, After Thomas, in which he played an autistic boy. Moretz started two years earlier, in a TV show called The Guardian, followed by The Amityville Horror remake. So why did they want to be actors?

Moretz: "My brother Trevor is theatrically trained. I used to watch him when I was younger and I was in love with it. It just seemed really fun to be someone else. So I begged my mom; she was hesitant, but she eventually allowed me. And it turned out well, I guess."

Butterfield: "I didn't at first. It was just something fun. But when I did The Boy in the Striped Pyjamas, that's when I thought it could be a career opportunity. Not many people get to have such an experience."

How are you looking after your money?

Butterfield: "Good. I've got pretty much all of it. It's all in the bank and I can't access it till I'm 18. But my per diems I have on my bank card. I don't know what's on my card just now cos I just got a MacBook Pro, which I'm quite excited about cos it's arriving soon."

Moretz: "Mine is all locked up. I don't really use any of it. My mom gives me an allowance. She keeps me pretty tight-reined. The problem with me is, anything that's easy I will just overdo it. Especially with clothes. But I'm 14 – my mom is super-strict about that. And, annoyingly, tries to keep me very, very, very grounded. It's smart, I guess. But it's odd – most 14-year-olds don't get an income. It's not normal! So our parents are just trying to regulate it."

Butterfield: "At school, when we were doing life skills, they were talking about taxes. And the teacher said, 'When you get older you'll have to pay taxes – none of you have to do it yet.' And I'm like, ahem..."

And how are you spending your money?

Butterfield: "Apart from the MacBook, a projector and SurroundSound in my room. I recently went clothes shopping, down Selfridges and Harrods, and hated it."

Moretz: "WHAT? Oh my God, I have to take you to Bergdorf's, it's right here. It's the most genius store you'll ever go into. I love shopping."

How do your schoolfriends react to your work, and to your fame? Request autographs? Suck up? Wedgies?

Butterfield: "No wedgies. Few autographs. But they know I don't really talk about it outside the filming world."

Moretz: "No wedgies, just swirlies – when they put your head in the toilet and then they flush it, ha, ha, ha! But I [only] went to school till third grade. Then I was home-schooled. When I was in regular school, people got rude. A lot of teachers would get mad at me. One teacher gave me purposefully bad grades – even though my work was perfect – cos they didn't like me being an actress. They said I smiled too much. That was interesting when I was eight..."

We talk about the limits of what child actors can and should do. Moretz's mother wouldn't let her watch the 15-rated Swedish vampire movie Let the Right One In, on which Let Me In was based. But she was OK with her training with Navy Seals for Kick-Ass, and learning knife- and gun-handling. As to the film's infamous swearing – Hit-Girl says, "OK, you cunts, let's see what you can do now!" – she says that if she'd brought her potty-mouth offset, she'd have been in trouble.

"In my house if I ever cussed it would be really bad. But even though I was only 11, I was more mature than most 11-year-olds so I could understand that I was playing a character. That doesn't mean I can go home and shoot guns and kill people and cuss."

For the climactic moment in The Boy in the Striped Pyjamas, in which Butterfield's character dies in a concentration camp's crowded gas chamber, the film-makers opted to shoot the scene at the very end of filming, to give the 11-year-old time to get used to the idea. Still, "It was pretty terrifying. I actually almost threw up at one time."

Little wonder. It must have been intense, shooting in a confined environment, surrounded by naked men... "Semi-naked," Butterfield interjects. "Everyone had trousers on."

"Oh Asa, ohhhhhh..." ribs Moretz. "You loved that scene."

Butterfield rolls his striking blue eyes and tuts. "You are so immature, Chloë."

In terms of role models, did Chloë look at where it went wrong for Lindsay Lohan, and right for Natalie Portman? "Yeah. Ever since I was a child I always had a dream to be Natalie Portman or Audrey Hepburn or Grace Kelly. I never wanted to stray. I have a really good family behind me. My mom's downstairs but my brother's listening to this. It's always one of them listening! I got spies all over."

Asa, when you grow up would you rather be Daniel Radcliffe or Robert Pattinson? "Um, Radcliffe. Well, I don't know, cos Dan, for a lot of his life he's gonna be known as Potter... I want to be myself. Pattinson's got all the girls chasing after him, and Dan's got all the geeks."

One final thing: why do Americans spell things funny?

Butterfield: "Cos they're retarded."

Moretz: "Oh my God, that's so PC of you."

Butterfield: "PC? You mean it's un-PC?"

Moretz: "That's sarcasm. You should know that, Mr British."

Butterfield: "They say Nootella! What is up with that? You don't say coconoot. Or hazelnoot. So why is it Nootella?"

Moretz: "You have UK English or American English."

Butterfield: "That's just to do with spelling."

Moretz: "Whatever..."

And there we must leave them. A red-carpet premiere awaits. And, probably, some more name-calling.

'Hugo' (U) is on general release from Friday