It was a drastic decision, but there was no going back. Given the circumstances there seemed to be very little option. Professionally ruined, discredited by friends and family, and having lost everything he held dear, there was only one thing left to do. Slowly he walked forward from the stony beach and into the chilly dark waters of the North Sea, continuing until the water closed over his head and he was gone - apparently for ever.

And fade to black. As endings go, the final scene of the second series of the BBC1 spy show Spooks must have seemed pretty conclusive. But real life (and sometimes drama too) isn't quite as clear-cut as that. The apparent suicide of spy Tom Quinn was not to prove the end of the programme - a third series has already been broadcast. And for Matthew Macfadyen, the actor who played Quinn, the character's disappearance into the depths was as much a beginning as an ending. The surprise success of Spooks meant that by the time of his "death", Macfadyen was on the verge of becoming something more than just an actor. In the very near future he may well become a star.

Macfadyen, a man delicately poised on the cusp of fame, is certainly about to become a leading man. This month he hits the big screen in the role of Mr Darcy in the latest film version of Pride and Prejudice, opposite Keira Knightley (who is already well and truly fivepointed). It is an iconic role, and one which is bound to cause whole forests of trees to be felled to generate the newsprint required for the inevitable discussions of how Macfadyen measures up in his frock coat and breeches.

Knightley, who plays Elizabeth Bennet, has already confessed he makes her go weak at the knees, and there's every chance that reaction will be repeated around the world.

He might be a man at the eye of a gathering media storm, but Macfadyen has the sort of calm that can only be displayed by those who are in complete control - or who have no idea what is about to happen. With him, it turns out, it's a little from column A, and a little from column B.

"I don't know how I feel, " he says, before a long pause to sip his tea. "Excited? Nervous?

I feel faintly ridiculous when people talk about it. I don't really understand it. I've had a bit of it before, but the job is still the same - it's just the media side that gets bigger. I haven't given it much thought, but there does seem to be a really good buzz about the film."

Macfadyen is sitting in a cafe outside the National Theatre in the South Bank complex in London, where he has been appearing in Henry IV, Parts 1 and 2. He is appearing in the show as the young Prince Hal, alongside Michael Gambon as the tragi-comic blusterer Falstaff. He is seated - completely by chance, I might add - underneath a poster that features both him and Gambon, but no-one takes a blind bit of notice. This tall young man barely draws a second glance.

The simple fact is that Matthew Macfadyen does not stand out in a crowd. He has the look of a young, pleasant, engaging everyman. It was that look, or perhaps non-look, that gave him his big break on television in Spooks, where he played a man who was supposed to blend in. We, the viewers, knew so little about Tom Quinn that we weren't even sure if that was his real name.

Macfadyen has revelled in this anonymity, which has allowed him to ply his trade without fuss and become, at the age of 30, one of the country's most respected actors. He has famously only been out of work for three months since he graduated from RADA ten years ago, which is perhaps the best indication of just how well he is regarded within his profession. If all of the good things people are predicting for Pride and Prejudice come true, that professional regard may give way to fullon public adulation.

The only thing that could cause problems is his apparent (and welcome) inability to talk in soundbites. A thoughtful man, he doesn't always speak in joined-up sentences. His paragraphs are brittle and broken up with pauses as he weighs up his words. Part of it might be a slight puzzlement at the incongruity of the interview process - having to be interesting and engaging to the latest in a whole line of people you have never met before. Part of it, though, seems to be a willingness to engage in the process: to treat each question seriously and give it due weight before replying.

Macfadyen has the gift of listening. That may be why, when you look at previous interviews and articles, the words and phrases most often used to describe him are these: reserved, stoical, unassuming, ill-at-ease, wounded and misunderstood.

None of these, however, really describes the man sitting opposite.

He is tall and broad, and his clear, rimless spectacles give him a slightly bookish look, rather like a young lawyer. Once he relaxes he laughs easily, and it becomes rather like chatting with an amiable but occasionally earnest neighbour with whom you have a nodding acquaintance.

Macfadyen does, he says, have some experience of what fame is like. Spooks made him a star on British television, and its success both here and overseas made the BBC very, very happy. He could have played Tom Quinn until the series spluttered and died - but he left the show after only two seasons. There was no disagreement, no falling out; he had simply signed on for two series, and that's what he did.

In fact, he liked the show so much he agreed to appear at the start of series three to ease the transition to his successor as the lead male role, Rupert Penry-Jones.

"It wasn't hard to leave, no, " he says bluntly.

"It was very sad because we were a new company and I had made a lot of friends, but I wanted to go. There's only so much you can do with one part." While staying on would have given him the financial security rarely experienced by actors, he insists his motivation lay elsewhere. "It's not why I wanted to be an actor, to play the same part for so long, " he says. "I'm not in it for the money. If I was, I would never have become an actor - nor should anyone."

Spooks changed his life in a much more tangible way than mere success. It was on the show that he met and fell in love with his costar, Keeley Hawes. The problem was that Hawes was married with a small child, and when she left her husband, the cartoonist Spencer McCallum, to be with Macfadyen it caused something of a tabloid sensation.

"It felt quite bad at the time, " Macfadyen says. "It was harder for Keeley than for me. The whole thing was fairly unpleasant, but it was worse for her. There's no story now because it has all settled down. Unless I do something outrageous I'm not going to sell many papers."

Macfadyen and Hawes are married now and have a nine-month-old daughter - a sister to Hawes's son Myles. Macfadyen is very much the proud father. "She's perfect, " he says of Maggie. "Slept all night from six weeks."

His stint at the National allowed him to be there for those crucial first few weeks of his daughter's life.

He is, by his own admission, happy - something he obviously doesn't take for granted. Besides having Hawes and the children, he is doing what he always wanted to do. He doesn't recall a single doubt about being anything other than an actor.

As a child he led a peripatetic existence because of his father's job with an oil company.

He was born in Great Yarmouth - the Scottish surname comes from his paternal grandfather, who was from Glasgow but moved south to join the Celtic diaspora around Corby in the 1950s - and his childhood took him to Dundee, Aberdeen, Louth and even Jakarta before his parents decided he'd be better off in boarding school. He settled at Oakham School in the Midlands while his parents continued their nomadic life. Acting was obviously in his genes;

his mother was a professional actress turned drama teacher. But it was at Oakham that he really discovered what he wanted to do with his life. "There was a great drama teacher, Dave Smith, " he recalls. "I found acting to be such fun and so liberating, and I always got excited about the school plays. The school was very supportive - we even went to the Edinburgh Festival."

Even so, he knew acting was a precarious existence - his mother no doubt taught him that - so he kept his ambitions away from his schoolmates. "I hadn't seriously thought about anything else, but I knew it wasn't a profession where you stand on a table and broadcast your ambition to the world, " he says. "You'd have to be a complete numpty if you didn't know it was a tricky business. So I did hold back a little bit."

For Macfadyen, "holding back" meant auditioning in secret for drama schools. Even at a tender age there was no denying his talent, and he was accepted for Rada at 17. The average age of students in his year was "about 25". When he graduated in 1995 he found work straight away, and plunged into the world of bad digs and tiny theatres on a long tour of The Duchess of Malfi.

"I actually did two long tours, " he says. "The thing is, when you come back you expect everyone to be saying, 'Ah, here's this brilliantly talented youngster' - but because you've been on the road for so long, no-one really knows who you are."

Although that was barely a decade ago, Macfadyen can't help but notice things have changed. He is dismayed by the attitude of some young students emerging from drama schools now, and their desire for fame rather than a knowledge of their craft. In the era of Heat magazine, there seems to be a weary acceptance that exposure is everything. "It's all about celebrity now, isn't it?" he says. "We are in a climate now where you can be famous for just being on the box." He gestures behind him to the largest of the South Bank theatres. " The Olivier holds 1,200 people and it's been packed every night for this run. But millions more people see a single episode of Corrie." He spreads his hands helplessly.

The thought reminds him of an incident a few years ago when he was shopping in London and noticed a definite buzz about one of his fellow customers. It was Darren, one of the contestants in the original series of Big Brother. "The assistants didn't know how to approach him, " he says. "There was lots of pointing and whispering as he went around the shop. Fame now appears to be a legitimate career choice for some actors. It's about getting a telly series. I don't know, maybe I'm just shocked by this ambition." He stops his soapbox moment, realising he might sound terribly pompous, but the point is a valid one. "There is nothing wrong with being an actor to be rich and famous, but the chances of actually achieving that are very slim, " he says finally.

Macfadyen is a stage actor by choice, although the last few years have seen him work almost exclusively in television and film.

In his perfect world - the one where his six numbers come up tonight - he'd do nothing but plays. He would, he insists, rather play a great part on stage than "bag a part in a Hollywood horror film that's going to bring me more clout".

His next part might not be in a Hollywood film as such, but it will certainly get him noticed there. Pride and Prejudice opens here later this month, but it has a late-November release date in the States, which indicates that the studio sees it as an Oscar contender. The film's director, Joe Wright, evidently had to fight for Macfadyen as his Darcy, something that doesn't surprise the actor. Claiming not to understand the wisdom or logic of film producers, he says he was aware the battle was going on behind the scenes but didn't let that distract him from the process of getting the part. Presumably he was smart enough to realise that, with Keira Knightley as Lizzie Bennet, the film had enough marquee value to stand a Darcy who is relatively unknown in film terms.

Macfadyen hadn't read Jane Austen's novel before he was cast in this adaptation - and didn't open it until he had finished the shoot. Past experience has taught him that reading the source material is not always the way to go. He recalls that when he was cast as Felix Carbury in the BBC's acclaimed adaptation of Trollope's The Way We Live Now, he decided to read the book as preparation. The first thing he saw was a description of his character as "green-eyed and raven-haired", neither of which applied to him, so he shut the book and went back to the script. That was the same approach he brought to Pride and Prejudice. He also tried not to think about anyone who had played his character before - not so easy when you think of a certain Colin Firth, and in particular of a scene (you know, the one with the water and the shirt) that had the media and audiences in raptures. As Macfadyen says, though,

if you went into jobs with that approach, no-one would ever play Hamlet again.

"It's the nature of it. You just get on with it.

It's a wonderful part, " he explains. "I find Darcy very sympathetic; I find him sort of heartbreaking. He seems very haughty. I mean, he is very haughty and proud and all the rest, but he's a young man who's still grieving for his parents. He's from an ancient family and has this huge responsibility, and it seemed to me that he's still trying to work out who he is and how to be in the world. I find that really interesting. I just find it very sympathetic.

"Looking at it now, Darcy would seem much more snobbish in our understanding of the word than he would then. To somebody like Darcy, the Bennets would have seemed very vulgar. It would have been a big deal for him to get over the status and to be able to say to Lizzie that he loved her. We would think, 'How snobbish and elitist, ' but it wasn't for him and it would have been a big admission.

There is something of the ridiculous about Darcy because he thinks very deeply and seriously about things. He takes himself very seriously, as young men tend to do sometimes. So there is a bit of darkness which Lizzie punctures so cleverly." He shrugs, before adding: "I had a bash and hoped for the best."

Of course, Colin Firth's Darcy captured the heart of every thinking woman in the nation when he played the part in the BBC's awardwinning television adaptation of Pride and Prejudice ten years ago. There are still some who have a fit of the vapours at the thought of "that scene", in which Firth emerged from a pond with his wet shirt clinging to his manly torso. Both Macfadyen and director Joe Wright are both keen to stress there is no equivalent in this version. What the actor does admit, though, is that when he read the book, even after having just played the part himself, it was Firth he saw in his mind's eye as Darcy.

"The image I had was much closer to Colin Firth than to me, " he says, laughing. "But that's just the image you have. When I read Ian Fleming's novels I always see Sean Connery as James Bond. I do think the book transcends all that. It's such a young person's book, you can see her [Jane Austen's] youth and it is really charming. Colin Firth is a fantastic actor, and that adaptation rightly made him a household name because it was so wonderful."

We joke about "the curse of Colin Firth", in the sense that Firth is seldom able to conduct an interview these days without Darcy's name or that shirt being mentioned. But surely when you are an actor, and your profession depends on being able to dissemble and assume other identities, a role that defines you so completely is something to be avoided? Macfadyen thinks long and hard, as though he is considering that view for the first time. "If it does, it does, " he says philosophically. "I can't do anything about that. If it doesn't, it becomes just another part.

Either way, I can't sit and worry about it. A job's a job."

Macfadyen's run in Henry IV came to an end on Wednesday, and he is looking forward to a small break before the release of Pride and Prejudice. There's a film he'd like to do in Ireland, he says - but it would take him away from his family for four or five weeks. While the schedule for that is being worked out, his wife is working too: she has been in Jersey filming an adaptation of Thomas Hardy's Under the Greenwood Tree. There are, as he admits, a lot of deals being made - "if you do this, then I can do that" - so they can pursue their careers and look after their children.

He is savouring the calm while it lasts. The world premiere of Pride and Prejudice is next week. Then comes the UK release, then more publicity for the American release. But for the moment everything is fine. And whether his Mr Darcy heralds a bright film career, or whether he continues untroubled with a stage career, Macfadyen can at least describe himself as an award-winning film actor. He only found out a few days previously that In My Father's Den, a small drama he made in New Zealand, won him Best Actor in the inaugural New Zealand Screen Awards against a field that included such names as Sam Neil.

"I was stunned, " he says, laughing with delight. "I got a text out of the blue from a friend who worked on it to tell me I had won.

Apparently we swept the board - something like ten wins."

He admits that, given the distances involved, he's not expecting to see the award any time soon - but he's intrigued as to what form it might take. A statuette of a hobbit, perhaps?

Whatever it turns out to be, it's just one more thing to feel happy about. "Life's good, " he says, nodding with satisfaction. "It's interesting. I veer between apprehension, excitement, and indifference. But it's fun. It's lovely."

Pride and Prejudice is on general release on September 16.