The movie heroes made on MySpace

"Personally, I would've called 999 straight away if I found out someone was spying on me," chirps Myles. "But Hallam and Kate have such a strong connection. She can see through his crazy antics, and she realises that behind his off-the-wall behaviour he's a good person."

A vicar's daughter who grew up in literary circles in Notting Hill, Myles, 27, gives a coolly sexy performance as Kate that finally delivers on the early promise that landed her on the front of Vanity Fair four years ago, tipped as the next actress to fill the "English Rose" role. The movie that was supposed to make her career - the ill-conceived Thunderbirds film - was an abject flop, despite Myles's slyly witty turn as the cut-glass Lady Penelope. Three years of minor roles followed, including small parts in the action movie Underworld and Doctor Who (she met current boyfriend David Tennant on set in 2005).

Hallam Foe should save her from this mediocre run - which is perhaps why she was so keen to land the part of Kate, personallywriting to Mackenzie after she had read his script to request an audition. "I absolutely loved what he'd written. On the page it was terribly dark."

Mackenzie's greatest achievement is in rendering such off-key material with such levity. The Scottish director, 41, who shot his full-length debut, The Last Great Wilderness, on hand-held cameras, specialises in weird love stories. He followed the sordid Young Adam with Asylum, the tale of an asylum-keeper's wife in the north of England who has a violent affair with one of the inmates, for which Natasha Richardson won an Evening Standard best actress award in 2005.

Hallam Foe is more than just a dark romance, however, lightened by a certain devilish humour and a jangling soundtrack featuring classic Scottish guitar bands like Orange Juice and Franz Ferdinand (the latter wrote a new song especially for the film).

For Mackenzie, the key to ensuring the film's wide appeal was making Hallam sympathetic, even endearing. This was all down to Bell, he says. "Apart from being probably the best actor of his generation, I cast Jamie because of his sheer likeability. He brought so much to the part, and was so conscientious in making Hallam real."

One way in which Bell immersed himself in the role was to write blogs as Hallam on the film's MySpace page during shooting. "David encouraged me to do it," he says excitedly. "It really helped me get in character. And it's such a huge marketing tool that it would be stupid to ignore it. The internet is a big part of my own life. I don't even watch TV any more - it's just email and YouTube and viral videos all the time."

The tactic has paid off. Mackenzie first engaged with the internet community when he posted the screenplay for the film on the web. "I did that because I wanted to canvas opinion," he says now. "And because I'm mad." However, the cyber-chatter about the film began in earnest from the moment Bell posted his first comments as Hallam, with curious parties leaving their own rejoinders to the character's ravings.

Blogging has been so crucial to the Hallam Foe marketing campaign that there was even a special "bloggers" preview" of the film two months ago, though Mackenzie professes himself disappointed at the tenor of that audience's interest. "They all kept asking me boring questions about the blog," he complains.

It was rather different for the strikingly beautiful Myles, who found herself surrounded by would-be real-life Hallam Foes, only without the charm of Jamie Bell. "I have this line in the film - 'I like creepy men' - and all the boys seem to like it. At the preview, this very twitchy guy came up to me and said: 'Do you like creepy men?' I said, 'No, my character likes creepy men. I like normal, well-adjusted men.'"

The majority of young bloggers have identified with Hallam's preoccupations in the same way an earlier generation recognised the turmoil that Holden Caulfield was going through in JD Salinger's The Catcher in the Rye. As a Time magazine review said in 1951, "Salinger can understand an adolescent mind without displaying one". The same is true of Mackenzie, who reflects on the perilous state of the teenage mind without patronising it.

Myles believes this may be because Mackenzie retains a close affinity with Hallam. "I don't want to make too much of it," she says, her voice dropping, "but I think David was probably a lot like Hallam when he was younger."

Mackenzie will concede to some overlap. "My nickname at school was 'Weirdo'. Not quite sure why. And Hallam's a weirdo. But then everyone is weird, aren't they? We all go through that period at 17 or 18 of getting used to the skin we're in. Being an outsider allows you to re-invent the world on your own terms. It's good - originality comes from weirdos. Everyone else is just following the crowd. The future belongs to the weird."

Hallam Foe opens the Edinburgh International Film Festival on 15 August and is released nationwide on 31 August. www.haveyouseenhallamfoe.com www.myspace.com/hallamfoe