Don't photograph me from the waist down     The film hunk Colin Firth on set in Slovakia—what could be finer? Then we saw his trousers

Welcome to Slovakia. We've come to photograph Colin Firth and to talk to him about his latest film, Where the Truth Lies. There's me, the writer, and Martin Kollar, a Slovakian photographer with Tiggerish bounce and a quirky sense of humour, who surprises me by admitting that, until this week, he'd never heard of Colin Firth. By way of background research, Martin telephoned an ex-girlfriend working in London. She told him that Firth is known as a bit of a heart-throb. That was probably all Martin needed to know, but I added that Firth is believed to be a good bloke—not a precious movie star—and a fine and versatile actor. I may have said something about him being increasingly vocal as a campaigner, though I don't recall using the word "soapbox".

We're in Slovakia because Firth has come here to shoot yet another movie. Filming started early in the morning and it's not likely to finish until late at night. When Firth does finally become available, we'll only have a few moments to take his photo, so to be sure we get it right, we practise. We create a plausibly English environment, with lime trees and horse chestnuts. Martin asks me to be Firth, stands me on a soapbox, and holds a variety of framing devices behind me. The most effective is an opaque filter, borrowed from the film crew. The effect is stunning: sunlight brightens the filter so that I seem to shimmer. We agree that Firth will look fantastic. Hours pass, and the winter sun moves rapidly across the sky. Again and again, Martin adjusts the lighting setup as we are promised Firth will be with us soon. Every so often we wander over to watch the shooting, only to find Firth is central to every scene and can't be spared. Martin becomes less Tiggerish by the minute. Me, I bite my lip.

Only at the last moment, just before the sun drops behind a hill, does it seem that we shall have Firth for a few minutes after all. But there's a problem: his costume. We've come to talk about a film set in the late 20th century, and Firth is dressed like a Roman soldier. He dashes to his trailer to fetch something to wear over the top—but by the time he returns, a grim-faced assistant director carrying a clipboard and a walkie-talkie is waiting to call him back on set. The pictures will have to wait. The photographer, now playing Eeyore, resigns himself to losing the sunlight.

Then, just as Martin has finished dismantling his lights, Firth reappears, accompanied by the assistant director and a publicist whose exchanges with Martin have become increasingly embattled. We've got five minutes, they say grimly. Martin springs into action, moving the lights back where they were. Firth, wearing a sweatshirt over his leather shirt, climbs into position. But just as Martin starts to take some pictures, the publicist says we can only photograph Firth from the waist up. Otherwise, Sunday Times readers would see his Roman trousers and Timberland boots, also customised to look Roman. And we can't have that.

It's at this point that the artist loses his cool. He throws up his hands and says: "I can't do this. I can't make a hundred compromises in one day!" The artist in question is Martin.

Firth steps down off the crate. "I'm sorry about your compromises." He looks sincere, but it is impossible to rule out the possibility that he's also a little amused. Martin is already regretting his outburst. It's not Firth's fault, he says, and apologises effusively. Firth steps back up and Martin places me behind him, holding a screen of black felt. Look closely at the picture and you can see my fingertips.

It's not that Firth asks much, he says jovially. He doesn't demand outfits by Armani, for instance, or Hugo Boss. He's willing to be photographed with his hair and face smeared in glycerine — lending him a sweaty appearance appropriate to a legionnaire but less so to a movie star. He'd just prefer not to be pictured in trousers with a leather gusset, if that's possible. "You try to be a good bloke," he says, "and to make yourself available. But I don't want to open a magazine and think, 'Why on earth did I let them do that?'"

Many people might think it more embarrassing to be filmed in some of the off-colour high jinks of Where the Truth Lies: the sex, the drugs, the violence. Firth is unlikely to send still photographs from this to his aged aunts. Where the Truth Lies explores the dark and destructive side of showbiz success. Vince Collins (Firth) and Lanny Morris (Kevin Bacon) are the hottest showbiz partnership in 1950s America. When a beautiful young woman is found dead in their suite, their world falls apart. Fifteen years later, a journalist persuades a publisher to offer Collins $1m to collaborate with her on writing the untold story. Through a variety of conflicting accounts, the mystery is grippingly sustained until the movie's end. The sex scenes, featuring Bacon and Firth together in the same room as assorted young women, were anything but sexy, Firth reports. "We ran through the morning with our curlers in. Then we ran through it partially clothed, sorting out where the guy with the boom microphone was going to be. Hopefully, not anywhere near me: it takes all the sex out of it."

The film is based on a novel, in which Firth's character is American. The director, Atom Egoyan, changed that: he fancied that a Brit could be interestingly uptight alongside the unruly American played by Bacon. The models he had in mind for Firth included David Niven and Rex Harrison. As for the double act, Egoyan wanted Firth and Bacon to work up their own routine, and helped out by creating plausible environments for them: a club, a telethon studio. He even hired a professional laugher to react to their jokes.

"People have said that the act is crap," says Firth, referring to American reviews. "But if you go through the footage of the Rat Pack and people like that, a lot of their stuff was crap by today's standards. It was macho, often racist. I don't think our routine was crap—anyway, it wasn't about having the best gags. We were more concerned with creating glimpses of that world. It looks tired, like a proper routine."

The actors sometimes had to play both 1950s and 1970s scenes in a single day. "It was very, very bizarre. You're ageing 15 years, and then going back to when you are young and everything was going well for you—and then back again. But that is what actors do. I'm not complaining: I find it exhilarating. You get ready and they say, 'We're not doing that scene. The light's not right.' So you get ready for something else, then they change their minds again.

"People have this idea that if you don't get it right you can do it again. But you can't. Not unless you get 20 takes with Kubrick. Even then, it can get worse, not better. You don't want to piss everyone off because they all got it right—the other actors, the director, the lighting people, sound. And if you do ask to have another try, they might say, 'Sorry, but the scene coming up is even more important.' And you are thinking, 'Oh, so this is for ever?' That's something you catch yourself thinking all the time. I read a comment on the art of translation: you have never finished, you just abandon it. With film, that happens many times a day."

In person, Firth proves much as I had expected. Tall and good-looking, sure. A teeny bit earnest. And thoroughly polite: he goes out of his way to make himself available, sitting down with me three times to make sure I have covered everything. The first time, we speak for precisely 13 minutes. (The publicist times it, presumably so I can't complain later that I've not had long enough.) We sit on a bench in the freezing cold while the crew rearranges the set. Firth looks preoccupied, and answers questions defensively. ("I don't know how I choose the parts I play.")

The second conversation falls at the end of that day. Firth changes into his own clothes, then wanders back uphill to find me, rather than—as I'm sure he'd prefer—driving back to his hotel or to a particularly good sushi restaurant he's discovered in Bratislava. This time, he's more expansive. The publicist leaves us alone and we talk for more than an hour. I even get him to open up on the mechanics of acting, which I find fascinating, but which many actors prefer not to discuss lest they sound pretentious.

Firth was born in Hampshire, moved to Nigeria where his father was teaching, and returned to England aged five. He went to a comprehensive school in Winchester. As a boy he dreamt of being a writer. "But when I was 14 it came to me that I could act." So he spent two years at the Drama Centre in London, and landed his first role on the professional stage as the lead in the award-winning 1981 production of Another Country. After that, he joined the Royal Shakespeare Company. Firth's film career began in 1984 in Another Country, with Rupert Everett. In 1989 he took the lead in Milos Forman's film Valmont. His lead role in the TV drama Tumbledown, based on the Falklands war, earned him the Royal Television Society best-actor award and a Bafta nomination. He's since starred in the films The English Patient, Fever Pitch, Shakespeare in Love, Bridget Jones's Diary, Love Actually, and Bridget Jones: The Edge of Reason. In 2002 he was reunited with Everett in a film of Oscar Wilde's The Importance of Being Earnest.

But the part he's best known for remains Mr Darcy. More than 10m viewers tuned in to the BBC's 1995 TV adaptation. Worldwide, more than 100m have seen it. On Google, if you search for Colin Firth and Pride and Prejudice, you'll find about a quarter of a million websites. Most feature gushing commentary from women viewers, such as: "Oh, Mr. Darcy! How often in my reveries have I longed to console you." Or describing their all-time-favourite screen moment: "When Mr Darcy emerges from the lake (mmm Colin Firth!!!)".

The screenwriter Andrew Davies, who adapted the novel for the BBC, makes a point that's often overlooked: that Darcy represents a considerable acting challenge. "At the start, the actor mustn't give away too much the fact that Darcy is going to be a sympathetic character," he says. "But he must play him in a way that he's not just a really nasty person who turns into a really nice person." Firth's solution was to stay very still and to convey everything through Darcy's eyes. "I thought to myself, 'This is where he wants to go across the room and punch someone. This is where he wants to kiss her. This is where he wants sex with her right now.' I'd imagine a man doing it all, then not doing any of it. That's all I did."

Kevin McKidd, the star of the BBC's epic drama Rome, has been playing opposite Firth in Slovakia. Firth is "expected to smoulder all the time", says McKidd. "And his face, in repose, is a bit like that. But Colin injects fun into the work, and this business should be fun. He has the right balance between paying the job the respect it deserves and not taking himself too seriously." In this respect, perhaps having children helps. In 1989, Firth entered into a relationship with Meg Tilly, his co-star in Valmont. Their son, Will, was born a year later. In 1997 he married an Italian, Livia Guiggioli, whom he met while filming Nostromo. They have two sons: Luca, aged four, and two-year-old Matteo. "Your own children, by their very existence, make you rethink everything. They keep your mind alive and question you, and mock you."

Since meeting Livia, he's learnt to speak her language fluently—he once described that as the most romantic thing he ever did. He's also become heavily involved in cultural events at the Italian Institute in London. For his efforts, he was honoured in May as a Commander of the Order of the Star of Italian Solidarity. He'd see more of his family—and of the Italian Institute—if he wasn't so often abroad on location. Why not stay in England, I ask, perhaps do a spot more theatre?

"Theatre acting is infinitely easier than film, in every way. You don't have a scrambled process, you can't be messed around by the editor, you have proper rehearsals and you're on stage before an audience. But there are a million reasons, noble and otherwise, for doing film. The recognition is very high, for a start. And at its best, film is a beautiful medium. It is wonderful that you can keep it. To me, Spencer Tracy does not look dated. Nor does Garbo, even in her silent work. Film doesn't disappear into the air. I find it devastating that you lose theatre performance."

 Firth's not done much on stage since leaving the RSC, but he did win excellent reviews in a 1991 revival of Harold Pinter's The Caretaker, directed by Pinter himself. The experience evidently stuck with him. "When something is that well written, and the production functions that well, it gets under your skin." What was Pinter like? "Harold was extremely practical and economic. His notes were not designed to reveal what lay behind the text: they were to do with blocking. He might have said, 'Why don't you sit down before you say that?' And I couldn't believe how much that tended to solve the problems I was having. He also treated the text as though it were someone else's. He would make jokes about telling the author what he thought of it."

Firth was delighted when it was announced in October that Pinter had won the Nobel prize: "He's a stunning writer and very important." Is he equally impressed by Pinter's political views? Many people wish he would stick to play-writing, not attacking George Bush, Tony Blair and the war in Iraq. Firth curls his lip, assuming a scornful expression hardly less familiar than the trademark smoulder. "Why should he not have the views that he has? I don't understand this thing about celebrities 'telling people what to think'. It's an English whinge. If you don't like it, don't listen. I think Harold is courageous and passionate."

Firth had long been a supporter of Amnesty and Greenpeace, paying subscriptions, writing letters, attending protest marches. But recently he became more outspoken. "I didn't want the fact that I was famous to stop me, and maybe it was irresponsible not to use it." In April he met Supachai Panitchpakdi, the director-general of the World Trade Organization, to discuss an Oxfam report on rich countries forcing poor ones to open their markets, and dumping surplus crops. "I had the choice between being the guy who smiles for the camera and says a few words, or doing some homework and trying to make the issue my own," he says. "It's hard, when you open up that kind of door, to close it again and do nothing. You are open to attack. The cynics and those who don't give a shit are constantly on the lookout for hypocrisy in everything that might be well intentioned. Working with Oxfam, for example, and being tremendously well off could seem like a contradiction. It's a fair cop. But this is not all about giving away your possessions. We are all at risk of being called hypocrites."

He adds: "I have a huge distrust of certainty and conviction, particularly the crude type politicians like Tony Blair and Margaret Thatcher have, that people seem to admire so much. Where do they find that certainty? I don't believe it's possible to have that if you are an imaginative person."

Our third conversation takes place in a pair of folding canvas chairs in front of the director's monitors. Firth has put himself out for me and covered a great deal of ground. Among other things, I ask if he's ambitious ("I think I must be"), then about his long-standing interest in writing fiction and, more recently, directing. We also discuss travel and his attempts to play the guitar.

But as I sit at my computer in England, none of that seems half as absorbing as the photo session in Slovakia. The whole thing was over in just five minutes, but it seems to exemplify much that Firth told me about the process of making a movie: the exhilaration, the uncertainty, the dependence on good light. It even bears comparison, I like to think, with Where the Truth Lies. It didn't specifically address the destructive side of showbiz, nor was there a great deal of sex, drugs and violence. But there was a rather tired partnership, comprising Martin (the unruly one) and me (the uptight Brit). And there was ample scope for conflicting accounts of what occurred, like the ones used by Egoyan. Did Martin really throw a wobbly? Was the publicist a pain in the bum, or just doing her best in difficult circumstances? Was the real villain the assistant director with the clipboard and the walkie-talkie?

Just one thing's for sure. There was a ghastly crime, with an innocent victim. Not a naked woman dead in the bath, but a 45-year-old actor, a good bloke, Commander of the Order of blah-blah, captured on film in The Wrong Trousers, on a soapbox for all the world to see his horrid leather gusset. As they say, Mmm, Colin Firth! ! !