Overnight stardom won't change Coleraine actor Jayne Wisener. Even silly questions about Johnny Depp don't bother her, she tells Fiona McCann

'I was dressed as a boy when I first met Johnny Depp," says Jayne Wisener matter-of-factly, big eyes twinkling as she downplays her introduction to one of cinema's biggest heart-throbs.

The petite 20-year-old from Coleraine plays Depp's daughter in her first professional acting role in Tim Burton's Sweeney Todd, but she graciously laments the fact that she didn't get to meet him in the rather more flattering period costumes in which she is dressed for most of her on-screen appearances.

"It's not like I had the long blonde hair and the corset, you know, hoisting up my chest," she says, laughing. She recalls instead that when she and Depp first shared screen time, she was disguised as a boy. "It was like, 'Oh hi, I look like Oliver Twist, nice to meet you'."

It's an endearingly down-to-earth observation from an up-and-coming star whose fresh face lights up the screen in an otherwise dark film about a butchering barber in 19th-century London. It is clear that Jayne Wisener takes none of it for granted either, and despite being able to name-check the likes of Alan Rickman and Helena Bonham Carter as her colleagues, she still appears as starstruck as any other young girl from Northern Ireland would be if she were plucked from obscurity so suddenly and propelled into the fantastical world of big-screen stardom.

"It was one of those surreal moments in life that you just never forget," she says of the day she was told she was to play Johanna, Sweeney Todd's daughter in Burton's screen version of the Stephen Sondheim musical. During a summer off from her studies at the Royal Scottish Academy of Music and Drama, she was performing with the Music Theatre 4 Youth charity in Derry and was spotted by a talent scout. After a number of London auditions, she returned to her studies and was in the middle of a class when she finally got the call from casting director Susie Figgis telling her she'd been successful.

She dropped out of school - "It was great!" she says with the cheeky grin of a student playing truant - and went to work with Burton, whom she admits she had thought of as "a super-being or something". In the end, however, she said working with Burton actually helped take the pressure off her to some extent, as she was able to place total confidence in her director.

"Screen acting was hard, but Tim Burton just told me what to do and I was, like, 'okay'. I was in the hands of a genius! If I had done it badly, he would have said 'scrap that, do it again', so I didn't really worry too much," she recalls simply.

If Burton was intimidating, though, singing for the legendary Sondheim, who insisted on having a personal say in the casting process, would have been enough to try the nerves of even a well-seasoned performer.

"I think that was the worst I've ever sung the song," recalls Wisener of her performance of Green Finch and Linnet Birdfor the composer. "You get so worried that you're going to butcher it, really ruin it! You almost feel like apologising beforehand."

Wisener need not have worried. With her sweet, crystal voice, she sings many of her better known co-stars off the screen. It all appears effortless to the diminutive star, but Wisener insists with an ironic giggle that she suffered for her art when had to have her eyebrows bleached for the film.

"I was walking round looking surprised for half a year!" she exclaims with faux indignation.

PUT IT DOWN to her disappearing eyebrows or her sudden leap into the big time, but Wisener has plenty of reasons to appear at least a little startled by the events of her recent past. Surely, given the huge exposure her first role has garnered, Hollywood must now be crying out to meet her?

"I'd need to get my teeth fixed first, probably!" she says self-effacingly. "I don't mind funky teeth, but a lot of Americans prefer big white choppers."

It's this lack of pretension that is particularly endearing about Wisener, and she gives credit to her family for not allowing the hype to go to her head.

"My family just treat me like Jayne," she says. "I'm still annoying, and they still shout at me and wind me up."

She's also eager to mention her younger brother, John ("a very talented musician") and sister, Jillian ("an incredible singer"), both of whom, she claims, are set to follow in her footsteps. "We're the Von Wiseners!" she says.

For now, though, the spotlight is on big sister Jayne, and everybody is claiming her as their own. Wisener calmly declines to get involved in the politics of her birthplace, though, referring to herself as Northern Irish and refusing to pitch a flag on either side of the political divide.

"I'm not really into the whole politics thing," she says, although she does mention that she was a contestant in the annual Rose of Tralee contest, representing Co Antrim at the annual Kerry festival in 2006.

"I'm very proud of my country," she says, adding quietly: "The whole country."

Wisener is understandably more at home divulging stories about her co-star Depp than she is addressing the political situation in Northern Ireland. She speaks with glee of a moment at Sweeney Todd'sNew York premiere when she got to introduce Depp to her mother.

"He said to me, 'Oh here you are, you're my daughter', and then said to my mum, 'So you must be my wife'," she recalls.

It's doubtful that many of her peers in Coleraine will ever be able to tell similar anecdotes, and she admits that her newfound fame has been strange for some of her hometown friends.

"Most people are just very curious, which I can understand," she says. "They're like 'You've met Johnny Depp! What's he like? Do you have his mobile number?', and all these silly questions."

Luckily, journalists are paid to ask silly questions. So, do you have Johnny Depp's mobile number?

"Of course not!" she squeals. "I wouldn't be sitting here if I did, I'd be on the phone to him!"

Sweeney Todd opens in Irish cinemas on Friday