Leaving Harry behind for good

Biggerstaff also figured in the 2002 sequel, Harry Potter and the Chamber of Secrets. But he now admits the Harry Potter films have been not so much a magic wand to conjure up other glamorous parts as "a double-edged sword", prompting interest among some producers and hostility from others.

"Some people love it, but some have a kind of snobbery about anything that is that mainstream - and the fact that those films were largely for children," he says. "Sometimes it will help that people know you from that, but sometimes they will discount you because of it."

Biggerstaff is 24 next month, though with his boyish good looks and slight build he could still pass as a teenager, were it not for the beard he has grown - or rather, been attempting to grow, for it struggles to cover his cheeks. He has worked only intermittently since Harry Potter - but he is hoping, with good reason, that a starring role in the film Cashback might boost his career.

Cashback, which has been chosen for the opening gala at the Glasgow Film Festival tonight, started off modestly as an 18-minute short, was nominated for an Oscar, picked up a host of other trophies and was extended into a full-length feature. Biggerstaff plays Ben Willis, an art student who has split up with his girlfriend, suffers from insomnia and takes a job on the night shift at a 24-hour Sainsbury's. Paradoxically, he finds that he can make time pass more quickly by making it stop altogether, and wandering round the frozen figures of colleagues and shoppers.

Readers of a certain age may remember an Australian children's TV series called The Magic Boomerang, in which the protagonist could freeze time by throwing his boomerang and thereby foil bank robberies and the like.

Ben, however, puts his ability to more interesting use, for the shoppers in his panoramas are not only frozen - they are also naked. It also helps that this particular branch of Sainsburys appears to be frequented largely by glamour models.

The film-makers had permission to shoot at night in the Whitechapel branch of Sainsbury's. "Even though it's closed at night the staff are still working, stacking shelves and stuff," says Biggerstaff. "They didn't know what the film was about and a guy would come past with a palette full of tinned tuna or whatever, and turn round and see all these girls standing with their clothes off."

The scenes in which Ben freezes time were done without computer technology, which Biggerstaff found a refreshing change after Harry Potter.

Biggerstaff thought that the original script was "cool and different", but there was never any suggestion of a feature film version when he first signed up. The feature has now expanded on this basic episode to encompass Ben's childhood and explore his adult life much more fully, including a developing relationship with another shop assistant.

It also gives Biggerstaff the chance to follow in the footsteps of the likes of Harrison Ford by providing the voice-over that holds it all together, so it is very much his film.

The young actor played the Duke of Gloucester in the BBC mini-series Charles II: The Power and the Passion, and appeared on stage at the Royal Lyceum in The Girl with Red Hair, which was written by Keira Knightleys mother, Scottish playwright Sharman Macdonald. But Cashback is his first film since Harry Potter, as well as his first lead role, so there is a lot riding on it. "I didn't feel any pressure making it because of the way it happened. It was a slow, gradual process, making a short and then doing the feature, and not knowing as we were doing the feature that it would be received as well and as it has been so far."

Cashback has already screened at several other festivals and has been sold all over the world. "I feel a little bit of pressure now that so many people I know are going to see it at the film festival," he says.

Biggerstaff still lives in Glasgow and has a flat in the west end. The son of a fireman, he began acting at school at the age of five, but does not count that as his start. "I hated school," he says. "In fact I failed drama. The only thing that I ever failed at school was drama."

He joined Maryhill Youth Theatre at seven, for "something to do on a Saturday", and made his debut as Tiny Tim in A Christmas Carol.

At Scottish Youth Theatre he was spotted by Alan Rickman, who was looking for a couple of boys to cast as truants in his 1997 film, The Winter Guest. "I remember he sat at the back of a class one day when we were doing general Saturday sessions, which were brilliant. We were acting out sketches and then I guess he just had a word with the teacher."

So the young Biggerstaff found himself rubbing shoulders with Emma Thompson in Fife, though he actually made his screen debut the previous year as a 12-year-old in the BBC adaptation of Iain Banks's The Crow Road.

The Winter Guest was also written by Sharman Macdonald, who promised to write something specifically for Biggerstaff. He has also met Keira Knightley, though there is no suggestion at the moment that Macdonald might write something that would bring the two of them together.

"I would love to do anything that Sharman wrote and I would love to work with Keira, so any suggestion of it would be very welcome. But it's not about to happen - not unless she's just not told me yet."

In the meantime, his next film may be an adaptation of a novel called Notebooks of a Naked Youth by punk rocker and poet Billy Childish. Biggerstaff would play an art school drop-out.

Despite his fantasies about nude shoppers, Ben in Cashback is a fairly wholesome young man. The debauched protagonist of Notebooks is "the opposite end of the art school spectrum", according to Biggerstaff. But the actor retains a great affection for Harry Potter. He keeps in touch with co-star Daniel Radcliffe, is looking forward to the release of the final film and is currently planning to re-read all the books in readiness for the big event.

• Cashback opens the Glasgow Film Festival at the GFT tonight. Visit www.glasgowfilmfestival.org.uk.
• Read Alison Rowat's review in today's Going Out section.