What can you tell us about Malachy's personality?
Malachy's a very sharp, very smart kid. When we first meet him he's doing a crappy summer job in the local leisure centre but he's doing it for very practical reasons - to earn money to go out with his mates - and he's obviously not going to be stuck there forever. He does well at school - I imagine he's one of those guys who claims he never studies but then ends up doing brilliantly at exam time. He's got his whole future ahead of him, as people keep telling him, much to his annoyance.

So he's got his head stuck on. But at the same time, there's a wilder side to him and that's what comes out when he's with Luke. And there's a competitive edge to that relationship. Though Malachy comes from a stable, warm family background - unlike Luke - he doesn't want to be seen as the sensible, well-behaved one. He's keen to show that he can be every bit as mad as Luke. And all of that comes to a head when they meet Michelle.

The film obviously centres around the breakdown of Luke and Malachy's friendship as they fight for the affections of Michelle, leading to "a deadly end". How did the concept for the film come about?
It was always a teen movie about the competition between two guys for one girl. Initially there was an idea that the competition would revolve around the idea of 'sin' - Malachy and Luke would try and outsin one another! But it quickly became clear that the language of sin, that kind of religious language that was drummed into me when I was growing up in Ireland, didn't really mean anything to kids of the age of Malachy and Luke - the fifteen/sixteen year olds of today.

So that idea was parked. But by that stage the characters themselves had taken over really, which is what you always hope for as a writer, so I just had to pay attention to them and let them kind of dictate their own story. And then when the directors, Lisa and Glenn, came on board, they brought a whole new set of ideas to the table, to do with the changing nature of the city, the leisure centre setting and so on, and their own kind of energy, so that all fed into the thing as well. It's a very organic process, writing a film.

Did Rupert portray Malachy exactly as you'd imagined and/or hoped? What do you feel he brought to the role?
The great thing about working with really good actors is that they take the character on and make it their own, to the extent that, as a writer, you almost can't remember how you'd previously imagined the character. So now when I think of Malachy I just think of Rupert. He really has made him his own. He's just such a fine film actor.

The years of experience he's had of working on the Potter films - I mean, he's an old hand now, he's a film veteran! - that stuff really tells. He knows just how much and how little to do - there are wonderful moments of stillness in his performance in Cherrybomb, for all the wildness and energy of the world around him. He's very subtle and expressive. I just love his performance and I can't wait to see what he's going to do in the future. he's clearly in it for the long haul. I'd love to work with him again.