Colin Firth has a habit of setting female pulses racing, so how will his fans respond to seeing him as a senior Nazi at the infamous Wannsee conference where the Final Solution was formulated? Is the move from ladykiller to mass murderer a step too far for the man for all seasons?

Colin Firth slopes into the room, filling it with megawatt charm. His Erect Highness extends a warm handshake at the door of his dressing room at Shepperton Studios in London. It is early evening and he has just come straight from a film set. Still in makeup and costume, he wears a double-breasted grey worsted suit and, heaven forfend, those gloriously peaked cheekbones have been painted powdery white. The deathly pallor suggests a corpse that has just been prepared by a cosmetologist for an open casket. The effect is entirely deliberate, of course. For his latest film, Conspiracy, Firth is playing one of Germany's most senior Nazis, who in 1942 attended a secret conference at Wannsee, a suburb of Berlin, to formulate a plan to exterminate the Jews. Being a film baddie obviously agrees with Firth, though it's a role that many of his legions of fans might not be too happy for him to play.

He set the female half of Britain on fire as the smouldering Darcy in the last BBC version of Jane Austen's Pride and Prejudice. How they will receive him as a Jew-killer might not be open to a lot of interpretation. But since he turned 40 this year, Firth, who is joined in this film by stars Kenneth Branagh, Stanley Tucci and David Threlfall, has become a risk-taker supreme with his choice of roles.

Dispassionately, he proceeds to nutshell the Conspiracy plot. He says 'Some senior Nazis, 15 to be exact, in January of 1942 met and discussed the extermination of the Jews. They had a nice buffet lunch and went home. Minutes of the meeting have survived and this thing is based on those. It is shattering stuff. This is utterly banal. They cracked a few jokes. Discussed whether bullets were better than gas. Whether sterilisation was better than forced emigration. Basically, the brief was no messing around with these half measures. We have to free German living space, as they put it, from all Jews so there is not one left.'

No half measures for Firth then, either. Professionally, he feels he is about to peak and his forthcoming roles reflect that. He is playing Mark Darcy in the film adaptation of Bridget Jones's Diary which gave author Helen Fielding a worldwide publishing hit and convinced a generation of women that they weren't alone in their 'sad singleton' status. The literary phenomenon with an obsession for fat units and fags will transfer to the big silver with Texas-born actress Renee Zellweger taking the lead role, with Firth and Hugh Grant, playing Daniel Cleaver, the main focus for her neurotic affections. But first up he stars in Donovan Quick, a film drama for BBC1, directed by David Blair, and unimpeachably the best thing to come out of BBC Scotland in quite some time. It is a dark horse among dramas, a film with a political, social and moral conscience. He stars as the mysterious noble gentleman of the title, pushed to the edge of his sanity by an incident in his past, whose selfless actions ultimately change the fortunes of a family of pathetic basket cases headed by alcoholic landlady Katy Murphy. The script decrees that lucky Katy gets to play tonsil hockey with Firth, which will make her the object of envy among a few million of his devoted female fans.

This update of Cervantes' Don Quixote starts off as a humorous satire on the tribal rites of a filthy-rich Scotland-based corporation making a mint out of privatisation of the buses and railways, but leads to a very dark and bleak climax. Shot in early 1999, the film has been almost criminally neglected for more than a year by the Beeb's schedulers before being rushed with almost indecent haste into the Christmas programmes package.

But let's begin with a few first impressions about Mr Firth. The mere mention of his name conjures up images of a gentleman dressed in tantalisingly soaking-wet breeches and white shirt at the moment of Pride and Prejudice's 'pond scene'. As a result, he draws out a nanny response from some women. Not having access to Dr Freud's emergency hotline number, Mr Firth has never quite worked this one out. It is suggested he might have been exposed to one too many easily flustered Bridget Jones types among the journalists who are dispatched to interview him, mostly tedious fans it seems, who lasciviously drool at the bottom lip and reverentially regard him as a potential between-the-sheets Apollo and Hercules all rolled into one.

Which surely sometimes must make him wish that he had never set eyes on Mr Darcy. On the contrary, he says: 'This idea that I have fled from it or rejected it or am uncomfortable with it, I don't think I have ever felt that. And where do you get this idea about tedious female fans? There is no such thing. I like female attention. I think it is fantastic. I don't want women to think I am standoffish. If somebody finds me attractive I will take that. I love it. I am not horrified by it.'

Paradoxically, he became the focus of female desire at the time when he became involved with beautiful Italian producer Livia Giuggioli, now his wife. 'Interestingly enough that whole heart-throb thing hit at the exact time when I met my wife,' he reflects. 'There were two things going on at once. It distanced it slightly because I was going into a relationship which was going to turn out to be completely stable and permanent. Basically, I was being kind of stabilised at the very moment when this potentially destabilising thing was coming along.'

But to fall for a producer, someone in the business, wasn't that a bit . . . er . . . incestuous? 'It was quite deliberate to choose someone in the business, though she is not an actress,' he replies. 'It is very helpful that it gives us a connection - reference points that are in common - though doesn't give us the direct conflicting ego that it might if we were doing the same profession.'

This is a veiled reference to his previous two failed relationships, with his Valmont co-star Meg Tilly, with whom he has a son Will, now 10 : and Jennifer Ehle, with whom he fell in love when she played bright and witty Elizabeth Bennet to his Mr Darcy in Pride and Prejudice. But if costume dramas haven't exactly made him lucky in love, they have been good career moves. In 1996 he won several awards for Pride and Prejudice, and his later role of Lord Wessex in Shakespeare in Love.

Alas, the Bafta nomination for Mr Darcy did not bring the treasured prize. The disappointment left a bitter aftertaste. 'Such awards are a pleasant little badge to get at the end of a working year,' says Firth. 'I have never got any gongs that have changed anything for me at all. I think most thinking people are at the very least a little bit sceptical of their value in real terms. Most people, if they are honest, feel good if they get one and are disappointed if they don't. This is going to sound a bit earnest and not with the times. But I wouldn't be disappointed to lose them to be honest - the whole awards system, I mean. I was shocked the first time I was nominated for a Bafta. I was convinced I didn't give a shit until I lost it. I was very surprised. I kind of thought I was being very cool not showing up to the ceremony. I didn't do that as a gesture. I was working abroad at the time. I didn't realise until somebody phoned up and said they had given it to someone else. I said, wait a minute, that is not the script. I think I secretly had hoped for it.'

Yet here sits an actor who doesn't read his reviews : he measures them. He plays all his roles like a quiet bonfire. The critics adore his performances that have ranged from the homosexual Guy Bennett in the stage version of Another Country, to the part as the pilot cuckolded by Kristin Scott Thomas in The English Patient, to Nick Hornby's Arsenal-obsessed hero in Fever Pitch.

Previous writers have tended to paint him as some kind of perfectly nice English marvy-poo chappy with a toplofty way of speaking and a knit-sweater, stoke-the-coal-grate charm. But let's cut to the chase - Firth has the kind of charismatic good looks that give women delicious dreams wherein he swings to them on a hanging vine in a loincloth no bigger than Moshe Dayan's eyepatch.

At this he chuckles graciously. 'It seems an exciting time professionally. I am very lucky. I am not a frustrated actor. I sometimes wonder what the catch is. I do find it is getting more interesting with age. I do find the roles are getting a bit more interesting. I think that I have got a little bit more experience to bring to it all. Anyone who thinks I haven't got a wrinkle hasn't looked very closely. I feel quite proud of it. There is a feeling that as we get older we earn the wrinkles, and whatever we have got, whether it is a belly or the grey hairs.'

With Bridget Jones, success is virtually guaranteed. Fielding's Mark Darcy was based on Firth's television appearance. She has said: 'Mark Darcy is the nearest I came in the book to writing a character with a real-life human being in mind, ie Colin Firth as Mr Darcy. So I'm completely thrilled he's agreed to do the part.' She said he had all the 'suppressed emotion and raw pulsating passion' the character needed. Scripted by Notting Hill's Richard Curtis, it's got the big money-boys at Miramax and Universal as chief backers. Which means Hollywood might next come a-knocking on Firth's door. Does he have some lamentation about the prospect of working in Hollywood?

He says: 'I think I am just as healthily wry about it as most of my peers. Not jaundiced enough to turn my back on it if they come begging me to accept their dollars and all the perks that go along with it. I think I might have adopted a posture 10 or 15 years ago when I might have hoped that I would resist such things, but I don't think I am that puritan about it if it were to happen. The only time I am wistful about seeing the corporate, big business taking over the big stuff is when it is good.

'When I know there is a lot of money riding on it and there are executives flying over from LA to check up on things I feel greater pressure. I can feel fear around me when that happens. I think people handled Bridget Jones very coolly. But there are things in the past where the more money there has been, the more neurosis there has been around. I find it rather oppressive. That's the stakes being high in the wrong way. It doesn't fire me up. I feel more comfortable in a more independent atmosphere where the only people we have really got to answer to for the time being is each other and the people who are making a film. There isn't a corporation showing up saying: 'We need another love scene.' All the stuff which is actually not between the team who have actually got together to create this. I feel excited by taking risks.

'I don't know how much people understand what I mean here - I am not dropping medical supplies into a war zone. I think one does feel that the stakes are high personally when you perform. You basically have no right to any mercy when people judge you. You always do feel that sense of quite high stakes if you are going to go into untried territory like Donovan Quick. It is not that often that you read a fantastic script.'

As much as Donna Franceschild's writing worked wonders for Firth, a modern-day interpretation of Don Quixote, with sidekick Sancho Panza portrayed with a truly knockout performance by learning disabled actor David Brown, must have sounded like a supremely cockeyed piece of work when it first appeared in script form. Obviously an actor of stature and power was needed to bring all the unflinching nuances of the man to the screen in a believable way.

Donovan is half-clod, half-poet, and the effect of his power and sensitivity is scalding. He is a fourteenth-century gallant lost in the revolution of 21st-century callousness. He arrives at a Scots boarding house and befriends the landlady's slow-witted brother who can no longer attend day school because the transport company, without consultation, have changed the routes.

Donovan commandeers an old coach and suddenly the pair are in business in a David v Goliath struggle against the big transport boys. Murphy, in the best performance of her career, gives a brilliant turn, shrieking her lines with desperate beak-like movements, a woman scorned by too many clouts on the chin by life.

Firth says: 'It is so full of paradoxes. I think life is made of that. Is Donovan brave to take on the might of a big transport company or is he stupid? Is he gallant and noble or is he ridiculous and absurd? He is a walking contradiction.' In the script Firth has some memorable lines about standing up to the corporate bully boys: 'Resist them -stand up to them.'

Noble words for an actor to spout - but when was the last time Mr Precious Thesp Firth had to stand up and be counted in such a way? 'I am not free of the bully,' he asserts. 'Whatever you do and whatever level you are at, I think it is very unlikely that you are going to live your life without there being a version of it somewhere. There is always going to be a bigger fish. There is always going to be corporate interests which are inconsistent with your own dreams. Certainly as an actor in my position, many of the kind of things I would like to do are the preserve of big studios who have first refusal on scripts and have a box-office consideration on casting. I won't get a look-in on the level that is above me. I will see films that I am involved with and I'm making - and this is even worse - being controlled to their detriment by executives and being hijacked by people who really have actually very little to do with the process.'

So, there is a bit of the missionary in the man whose three grandparents were missionaries in India. His parents travelled as teachers. 'My family travelled a lot and dealt with people directly. They were not converting people. One grandfather was principal of a theological college. Another got a medical training late in life and went back as a doctor. The travel and the experience is something that has been an enormous privilege. Things were discussed. Books were around. We weren't wealthy. My privilege was not material but definitely in terms of communication and expression. It was quite free and vivid.'

His father was a history lecturer, mother a lecturer in comparative religion, so there was tension at the time when he was at school in the US and latterly a comprehensive in Winchester because he just wasn't interested.

'My parents had followed a path which had delivered something for them. They were afraid of what might happen if I didn't go that way.They weren't sure if my resistance to academia was nothing but an excuse. Whether wanting to be an actor was a genuine vocation I was announcing to the world or a cop-out. I didn't like school. I didn't like what was being imposed on me. I found it very difficult to do anything that I wasn't inspired to do and that, I dare say, is a weakness in me. It didn't work that way for me. I had to be into it.'

His first performance was as Jack Frost in a Christmas panto. He has vague memories of wearing satin leggings and a polystyrene crown. Then a yearning to do drama kicked in for real. 'My parents are delighted. Their worry was fear. It wasn't a desire that I should do what they thought I should do or that I should follow them. That was never their agenda. They were enormously tolerant. Any parent would be nervous if their child announces that they want to be an actor.

'I remember very clearly the first relief I saw with my parents was when I was at drama school and I was firing off it. I was into it. I was stimulated. It wasn't just some lazy option. They actually saw me in an institution for the first time, coming home saying it was fantastic.'

They ought to see him here now, on the set of Conspiracy. Seated at a table. Playing one of Hitler's henchmen. The SS eagles and swastika armbands abound. He and his actor pals spend all day sitting in this room while the cameras record them discussing unspeakable things. So how does he compensate for this greedy swallowing up of all these dreadful words and images from one of our worst periods in modern history? Can he remain unscarred by it, simply sauntering off to the comfort of his up-market home in Islington where he can distance himself from what has gone before?

He replies: 'Oh, you can't take on all of it all of the time. There's actually a lot of humour abounding. I think it is a release of tension. In some ways you feel a terrible phony. I think all the angst and rigour of the job is terribly enjoyable. It's fun angst. We don't really suffer.

'There are some dangers if you are not careful. It can mess around with your head a little bit. If you are not careful about keeping your eye on that, you can be taken by surprise by it. We usually find that the more tortuous it is, the more irresistible it is. I'd love that to be clear when we are talking about risk. This is relatively speaking. I am definitely aware of the good fortune I have. I find this a fairly exhilarating time in my life.'