I'd like the new Pope to see this film: Stephen Frears

Stephen Frears is home alone, pottering barefoot around his cheerful, art-crammed kitchen in slacks and a crumpled white T-shirt. He hasn't shaved. Would the photographer like him to go and shave? "Would he like to go and shave?" I counter. "I don't give a monkey's," he growls, in a tone dripping with comic resignation."I don't care. But there are times when you think, I can't go on looking like a slob."

Attire notwithstanding, on this quiet Saturday afternoon in Notting Hill, Frears (who shows no sign of slowing down at 72) is very much in work mode. His next film, which covers the cyclist Lance Armstrong's fall from grace, starts shooting in three weeks, and he is now, he explains, a fully paid-up member "of the School of Armstrong Studies. 

I've been thinking about him since last December and learning more and more. Every time I meet people, I say: 'Do I sound as though I know what I'm talking about?'?" He looks briefly worried. "I hope I do." Frears is one of Britain's highest-profile directors, but his work remains curiously hard to pin down. Starting out at the BBC and LWT in the glory days of Seventies drama, he has since taken whatever directing jobs he found most interesting at the time, resulting in a body of cinema and television that takes in almost every genre apart from outright sci-fi. "People like me go where the work is good," he says with heavy self-deprecation.

Often this scattershot approach results in blazing successes - Dangerous Liaisons, My Beautiful Laundrette, The Grifters, The Queen - but there are also, he admits, "some shockers", such as the overheated Jekyll and Hyde drama Mary Reilly or last year's rom-com Lay the Favourite.

It's equally difficult to classify his latest film, Philomena, which manages somehow to crossbreed a sobering tale of ecclesiastical corruption with an odd couple road movie. Based on a book by the journalist Martin Sixsmith, it tells the story of Philomena Lee, an Irishwoman who decides in her late sixties to trace the illegitimate child she was forced to give up as a teenager in one of the Irish Catholic Church's notorious Magdalene laundries, where unmarried mothers were incarcerated and subjected to hard physical labour in return for their "asylum".

So far, so unpromisingly grim - but the film, which stars Judi Dench as Lee and Steve Coogan as Sixsmith, leavens its bitter story with a large helping of comedy. Co-written by Coogan, who brings to the script what Frears calls "his strong sense of the ridiculous", it evolves into the year's strangest buddy movie, as Coogan's prissy metropolitan journo sets out across America, accompanied by Dench's warm-hearted, daffy pensioner and hounded by a barking magazine editor in London ("Who are the goodies? Who are the baddies?"), played by Michelle Fairley (best known to many as steely matriarch Catelyn Stark in HBO's fantasy series Game of Thrones).

Philomena won the award for best screenplay at the Venice film festival, but other plaudits received there appealed more to Frears's sense of fun. "What was particularly satisfying," he says, "was that we got an award for the best Catholic film and an award for the best atheist film. That made me laugh a lot." The mounting frustration of Coogan's lapsed-Catholic character also went down well in Venice with his explosive howl of "F------ Catholics!" garnering, Frears says, "a big round of applause". He gives a wolfish grin.

"I kept saying, 'Can't we repeat that line?' but in the end Steve came to me and said, 'I can't say it twice in front of my parents!'?" Frears isn't at all keen to talk about the politics of the film - "I'm Jewish, I'm not religious at all and I wouldn't pretend to know anything about Catholic politics" - but he will admit that he was "quite startled to be making a film in which God appeared.

Or in which God was talked about. I hadn't expected that to happen in my lifetime." He told an audience in Venice that he'd like the new Pope to see the film, but he doesn't seem to like being reminded of that. "Well, it's not a very difficult thing to say, is it?" he barks. "It's quite an easy, liberal thing to say. I mean, the Catholic Church don't make it difficult to criticise them." He reflects for a moment.

"But my next line, when I was asked about this, was that the Pope seems a rather decent bloke." Frears studied law at Cambridge in the Sixties, and although he has described the experience as "a--- achingly boring", his watchful precision in interviews would make him a fearsome opponent in a courtroom. Questions get batted away like flies - "It's not like that,"

"No, that's completely wrong," "Good God, no!" - but considerable warmth, a sidelong friendliness and a bone-dry sense of humour lie just beneath the grumpy surface. It's interesting, I say, to see Coogan playing a journalist in Philomena, after his very public criticisms of the News of the World during the phone-hacking scandal.

"I was always trying to get him to say the line 'Well, maybe we should hack into their phones,'?" Frears agrees. "He could see how satisfying that would be, for some inexplicable reason." And isn't the Armstrong picture about to put another journalist centre-stage - this time the Sunday Times writer David Walsh, who exposed Armstrong's doping? "I know," he says, eyeing me beadily. "It's shocking. I thoroughly disapprove. Anyone would think I admired journalists." But Philomena also marks the latest instalment in a clutch of recent films inspired by real events (The Deal, The Queen, his HBO drama Muhammad Ali's Greatest Fight) that seem to suggest the closest thing in Frears's work to a trend. "People have been trying to find one for years and failing," he says tightly, then relents.

"No, even I have noticed that. It's an interesting subject. If you say to an audience, 'This is a true story,' they start 20 yards up the track. In other words, fiction has become harder to get an audience to accept than fact. I'm being absolutely serious. There are statistics to prove it."

He concedes that the docudrama forms that draw him are "quite an odd combination of accuracy and invention, but I find that very interesting and satisfying." He muses for a moment. "Truth is quite constricting, in a way. You endlessly see at the start of a film 'This is a true story'. Whereas I prefer William Goldman's line, 'Some of what follows is true'." Walking the line between fact and drama can have its disadvantages, though. "You can't, sort of, unleash your fury," he says, when talk turns to his portraits of Tony Blair in The Deal and The Queen.

"You actually bend over backwards to be fair to people; it's too easy or too silly otherwise. So you're overly generous, I always think." But Blair, clearly, continues to fascinate him. "What is astonishing is that the two people who have learnt nothing from Iraq are he and Alastair Campbell; they're the ones saying bomb Syria, bomb Iran." He gives a wry smile. "I was trying to construct a theory that he'd actually put an end to war, because no one will ever go to war again without consulting Parliament. It would be a very interesting thesis, that in order to promote a rather admirable cause he had to behave disgracefully.

It's not a theory I've ever heard him or Alastair Campbell advancing." That's a Frears film I'd like to see, I say. "Well, that was what The Queen was about, wasn't it? In order to demonstrate her sincerity [about mourning Princess Diana] she had to behave hypocritically." He chuckles.

"Particularly satisfying." He claims not to know the budget for any of his projects. "I never ask," he says. "I just see the white faces of the producers. And I can see what their problems are. Ken [Loach] is retiring, because he says you make a film, it does quite well and then you start from the beginning again - so your life is spent permanently at the bottom of a ladder. "Now, of course, I have it easy. How other people do it I don't know, but I know the struggle for money just drives you bananas."

Even so, he sees no need to sound the death knell for British cinema. "Maybe money is now so tight that you have to make a good film, because you have no other choice," he says. "I've always found poverty a source of strength." What keeps him going, he says, is the thrill of discovery. "I have no idea what'll turn up next," he says happily.

"I like the surprise. I remember reading the script for Dangerous Liaisons and thinking that I could quite happily spend the rest of my life watching this film, the story and the writing were so wonderful. And Hanif Kureishi put My Beautiful Laundrette through the letterbox!

All I said when I read that was 'This is very, very good, we should do it now,' and they all said, 'You don't want to do it, it's television.' And I thought - why would you not do it? Who gives a monkey's about the form? It's the content that's interesting."

Frears hasn't thought about life after the Armstrong film, but, he muses, "you never quite know where you're going. And then when you end up somewhere you say, 'Oh, I see, that was what it was about.' You're trying to find that out, really." How refreshing, I say, to find an established director so open to new input. "Well, what's the point of being someone who isn't open?" he rumbles with mock belligerence. "I'm a tart!" Philomena is released in UK cinemas on November 1