It gets dark pretty early in Prague a couple of weeks before Christmas - except this isn’t Prague: this is an airforce base some 50 kilometres to the south-east, in the general direction of Vienna. The Czech paratroopers who are usually based there are off keeping the peace in Kosovo.

It gets cold very quickly when night falls. To make things even cosier, a low mist is beginning to blot out the lights of the nearby village and a thin, icy rain is falling. Everyone on the Deathwatch set appears to be absolutely delighted with the weather.

“The European crew can’t understand these actors,” says Michael J Bassett, the writer/director of the film. “They’ve never met anybody who’d do this for a role. But that commitment has a price for me as a director because it means you have to really bloody know what you want and know why you want it.”

After a half-hour of following Bassett round the 400 yards of very authentic-looking trenches that production designer Aleksandar ‘Sale’ Denic (who worked on Emir Kusturica’s Underground, after which even Deathwatch must seem a doddle), I am in no doubt that Bassett knows what he wants and is something of a genius at persuading other people to give it to him.

The other people in this case include two English producers; a German tax-fund executive and two executive producers, also German; a Polish cinematographer; several dozen assorted Czech technicians who all appear to shop at the same woolly-hat store; and a dozen of the best young actors in British film and theatre, led by Jamie Bell, making his first feature since Billy Elliot.

Bell, it turns out, had been reading the screenplay as it evolved, thanks to Billy Elliot director Stephen Daldry’s links with the production company. “I got many, many versions and it changed dramatically, then it changed dramatically again,” says the young actor next day in the artificial splendour of Prague’s spanking new, Libyan-funded Corinthia Towers Hotel. “It was good because the thing that interested me the most was that it was the First World War.”

After the enormous success of Billy Elliot, Bell was, of course, inundated with offers. “But,” he says, “they were all kind of similar: American kids’ roles or English kids’ roles – stuff that I really wasn’t interested in.”

Like most 16-year-olds, meanwhile, Bell definitely is into horror movies, although he prefers the classic variety to the post-modern - hence one of his reasons for wanting to do Deathwatch. “I don’t really like Scream and stuff, but more the weird horror movies, like here, where you don’t know where the problem’s coming from,” he says.

producer Mike Downey on location in the Czech Republic with Bell.

Bassett is particularly pleased both that the young actor wanted to do the film and also that he agreed to play the pivotal part of Private Shakespeare who, as the youngest member of Y Company, plays that classic horror-movie role of discovering things simultaneously with the audience.

Bell, it turns out was (like Bassett himself), something of a WWI aficionado. “It’s something I really enjoyed. Just the whole idea of the trenches,” he explains. “You know from research that there were boys younger than 16, and they probably really were scared of dying and being there.”

Although Bell plays a 16-year-old in the film, it is really his first adult role. “It’s difficult to make the step from child actor to adult actor,” he says. “All the scripts that were coming in were kind of kiddie scripts and I wasn’t really too sure if I should do that. I wanted to step over from that. This was a kind of child role, but with a lot of growing up. It was good, very good.”

Jamie Bell as Pvt Shakespeare in the trenches with Sgt Tate (Hugo Speer)

But the shoot, he admits, has been hard, “especially when it’s raining and everything. It’s very difficult to keep on acting when the rain’s coming down so hard that it hurts, and the mud’s so thick.”

That, of course, is just what Bassett wanted. He is inordinately proud of his trenches - and with some justification. They are up to 10-foot deep, snagged with barbed-wire (some fake but quite a lot, as I soon discover, very real), littered with dug-outs and dead dummies, and ankle-deep in mud. Much of thus turns out to be water-truck-aided, since the incessant Czech rain didn’t quite produce the required Flanders quagmire.

The trenches are also, quite literally, a maze. Admittedly it’s dark but, after a minute or so of following Bassett on his guided tour, I have no idea where I am. Bassett, already in seventh heaven because of the rain and the mud, is rendered almost speechless with joy when I reveal this.

“The trenches are a metaphor for death,” he explains happily. “Above is the potential for life; here is limbo; and below that is pure burial. That’s why the movie ends up in catacombs that are supposed to be beneath these trenches, because that is pure death. “One of the producers described it as a ‘shell-shocked dream’, which I thought was just wonderful. But it’s not a poetic piece. It’s not an art film. It’s a commercial horror movie.”