Asylum. With added glamour

They're the people we don't see – because they get out of bed before us, or because we simply don't bother to look up when they empty our bin or hand us the tab. But they're there, being bussed out to the fields on raw mornings to pick our vegetables, lugging the Hoover over our office carpets, frothing our cappuccinos, rescuing us from town in the early hours for half the price of a black cab. The editorials of the Daily Mail will tell you that the presence of asylum seekers and economic migrants is propelling Britain towards some kind of social crisis. As Stephen Frears's new film demonstrates, these people – unregarded and underpaid – do the work that keeps the country running.

The protagonists of Dirty Pretty Things are a pair of asylum seekers working, permitless, in London: Senay is a young Turk who plumps pillows and performs loo-paper origami in an outwardly respectable London hotel; Okwe is a Nigerian doctor who, when he's not turning a blind eye to the prostitutes tripping past the same hotel's reception desk, is chugging a minicab to Heathrow and back. Ken Loach went to Los Angeles to make Bread and Roses (2000), his film about the Mexican immigrants who mop the floors of California on slave wages and with no basic rights. Stephen Frears – the man responsible for My Beautiful Laundrette (1985), The Grifters (1990) and High Fidelity (2000) – has stayed at home, and produced a movie that offers a glimpse into a more proximate realm of exploitation: the illegal underworld of British service culture. So why did we hear him on the Today programme a couple of weeks ago, denying that his film was anything more than a simple thriller? "Well, all right, guilty," he pleads. "If you try to turn me into a serious person I'll be frivolous. It's a sort of irredeemable hooliganism. I'm just a bloke who makes films, so to suddenly be thought of as a person who's thought about these serious issues is just embarrassing. You already get away with murder being a film director. To be thought to know something about something makes me feel even more shifty." A director, I suppose, can only enjoy a limited amount of piety when he's being interviewed in the cosy library of a chi-chi London hotel, and has just asked the waiter – about whose legal status he has no knowledge – to bring him some smoked salmon for his lunch. But some film-makers relish the role of "conscience" and become advocates for the causes illuminated by their films. Lukas Moodysson, for example, whose new film Lilya 4-Ever exposes human trafficking between the former USSR and Western Europe, is currently preparing to distribute hundreds of videocassette copies of his films to sex workers' support groups in Tajikistan and Uzbekistan. For Frears, the uncovering of a hidden subculture was only part of the game: he also wanted to take two characters from this hidden world and place them within a thriller plot at which Robert Donat and Madeleine Carroll would not have thumbed their noses.

Which is why he cast Chiwetel Ejiofor (who has just opened as the coke-bibbing, lounge-lizardly Nicky Lancaster in the new Donmar Warehouse production of Coward's The Vortex) and Audrey Tautou (the Parisian poster-girl of Amelie) in the lead roles.

"It seemed to me that the characters deserved to have a glamorous film made about them," reflects Frears. "If I was a Turkish immigrant I would like to be played by Audrey Tautou. People want to be shown in a nice light. In my experience they don't want to be shown to be boring or miserable or victims.

"I was always encouraging everybody to be as glossy and Hollywoody as possible." Some audiences are made uncomfortable by such an approach.

American reviewers of the Eminem movie 8 Mile, for instance, griped that Kim Basinger is far too good-looking a performer to be playing the rapper's beer-slugging trailer-park mom – as if the cash-strapped could have their high cheekbones and straight teeth repossessed along with their HP washing machines. The same comments might well be made of Tautou and Ejiofor, but Frears has no sympathy with such views. He wants to focus upon the heroic, romantic qualities of migrants.

"All these people who come to England and then do these shitty jobs," he reflects, "quite a lot of them hung under trains and came in off the backs of lorries. They've crossed the world. I haven't. How can you pretend to be more interesting than people who've led such interesting lives? People wouldn't do it if they weren't in a mess. People don't do it to sponge. Their lives are terrible. We ought to be flattered that it's England they all want to come to." This is the nearest Frears will come to a campaign slogan. And, sensing, perhaps, that he sounds like he might be making some claim to moral authority, he tells me a self-deprecating story. After he had finished shooting the film, he suddenly became aware that he'd never discussed it with the woman who does his cleaning. She is a Russian from Lithuania with her own remarkable backstory – a story about which, he confesses, he'd never bothered to ask. "I've made a film about..." he began, but she cut him short. "I know," she said. "I read the papers."

'Dirty Pretty Things' is out on Friday