"Last time we met you may have noticed me looking at you strangely," the email said when Colin Firth went online one morning. "This is why," continued the email, going on to offer him a lead role in a film.

Trouble was, the email was not from any film director of whom Firth had heard. It was from fashion designer Tom Ford. "I didn't know what to think," Firth admits. "Like everybody else I thought, 'Isn't he to do with the fashion business, eyewear and all that sort of thing?' " Firth's scepticism was not unfounded; panic, in fact, might have been an appropriate response.

At the fashion house of Gucci, Ford was not known for his subtlety. His notoriously provocative advertising campaigns were explicitly about sex: carnal, animalistic, unsettling. Portraits of people about to have sex or who have just had it. Hints of violence and discord between naked women and nattily besuited men. Skanky and sleek seemed to be his signature.

Firth could only have been alarmed at the prospect of being invited into such a hyper-real, distorted world. He could have found himself stuck in some kind of hellish design concept, a vanity project photo shoot directed by a man whose reputation rested on flagrant attention-seeking.

"But when I read the script it was clearly not an opportunity for a series of visual delights," Firth says slightly wonderously.

"The story was about a lot more than that. When I met Tom properly to talk about it, I was convinced it really was a story he wanted to tell. This story was very important for him. And then it became a story I wanted to tell."

As it turned out, the American designer brought out a depth, sensitivity and radiance we have not seen before in this naturally reserved Englishman. His finely nuanced performance as George Falconer, a gay college professor living in Los Angeles in 1962 who is mourning the death of his partner, is so full of emotional intelligence that it won him the best actor award at last month's Venice film festival.

The elegiac story was adapted from Christopher Isherwood's novel A Single Man, written in 1964. It was praised at the time for its artistry but also denounced for its then unpalatable subject matter.

The narrative follows George through a single day, one he has decided will be his last. Before the light goes out he suddenly starts seeing rapture and poignance in every small transaction; every sense is enhanced and enlarged. He experiences all the beauty in the world that we forget to see as we hurtle through our daily lives.

As Firth enthusiastically explains, "He is given a gift of a last day on earth. Seeing everything for the last time, facing death without fear and, as he says goodbye to everything, he rediscovers life and falls in love with it because of that decision. I think Tom's immense skill is that there is a vibrancy to everything that you don't get on a normal day in your life.

"He (George) looks at the child next door and she is more vibrant than he ever saw her before. The smell of a dog, the smell of perfume, the sunset is more intense than he has ever seen it. The beauty of the boy who tries to pick him up. These are all far more vivid and intense because he knows he is seeing it for the last time."

George's secret, as he travels through his final day, invites all kinds of irony and unasked-for opportunity in the unknowing people he encounters. "I could have played it in ways that were a lot more sombre than I did," Firth says. He refers to what he calls "the melodrama of suicide . . . It is very theatrical in a way. It is a big gesture to the world. There was a sort of self-mockery.

"The character of George is ironic, the book is ironic. There is a lot of humour in the book. There is humour in despair and all inevitabilities that we are afraid of, like ageing and addictions. I don't think the film is ever po-faced, and even though there is profound sadness in the film I don't think it is ultimately a sad film."

Walking into a tented room on the beach on the Lido in Venice, Firth at 49 is still an arrestingly attractive man. The curly hair is greying slightly at the edges. But there is a delicacy about him that sits at odds with his tall, burly build. His face is all angles and planes, transformed when he smiles, which he does often.

This is my fourth encounter with Firth and I have seen some of the light and shade in his character that Ford recognised and drew from him for A Single Man.

Although a stalwart of British films with commercial intention, he has never before been seen as an award contender, partly because he has never taken himself particularly seriously. Seen as reliable, with a good glower and a serious degree of smoulder, his career has lacked gravitas. "I am neither reliable nor particularly moody," he recently said in his defence on Britain's Radio Four.

Indeed, anyone who has spent time with Firth knows he can be very silly and extremely amusing. "The silly side of me is pretty dominant," he admitted during a long interview—that he giggled all the way through—in Luxembourg on the set of Girl With a Pearl Earring in 2003.

Since the notorious wet shirt scene in Pride and Prejudice, he has spent a large chunk of his career sending himself up in reprises of Mr Darcy in British comedies, most notably in the Bridget Jones films. He has also danced between low-budget drama (Genova, Trauma) and big-budget comedy (Love Actually), was ridiculous in St Trinian's, clowning in Nanny McPhee, and veered into the mediocre, although wildly successful, film version of Mamma Mia!.

Of his performance in Mamma Mia!, Firth admits: "I enjoyed the singing much more than other people enjoyed my singing. One of my great tragedies is that singing is one of my great pleasures but it is best kept to the shower. But you know, whatever anybody thinks of that film, the reason it did so well, apart from the obvious factors ... was because we actually did enjoy ourselves."

He was filming Genova at the same time as Mamma Mia!, playing another grieving professor who decides to take his two daughters to Genoa, in northern Italy, in the hope of some kind of renewal. The experiences couldn't have been more different.

"Genova was a small movie because (director) Michael Winterbottom works in a very particular way. It was extremely intimate because there was no crew there, no lights, no security, no extras, no continuity person, no wardrobe person. There would be three actors, a camera, the director and a microphone in a room. They weren't constructed sets. And we were free to improvise our dialogue. We were just acting all day. If you wanted to do the scene again, he would take the camera somewhere else. He didn't care about continuity, you could change your clothes in the middle of it if you wanted to.

"And I was filming Mamma Mia! at the same time, so that was quite peculiar to then go from the Mediterranean sun to the rainy studio at Pinewood, and then inside to a false Mediterranean sun of Greece."

This kind of diversity means Firth has done a lot of stepping sideways, but not, until now, forward. "I just can't keep a straight face for that long," he says. "And I play a lot of roles which call for a straight face. I wouldn't be able to do serious stuff if I couldn't take a break and send it up again."

Firth, nonetheless, is one of those fortunate people who seems to be getting better, more supple and able, with age.

"The roles for men do get more interesting as you get older. Callow youth is really the dullest thing and the most difficult thing to play for an actor."

In Dorian Gray, his louche Lord Henry Wotton, the moral corrupter of the beautiful, innocent Dorian Gray, is a portrait of contained danger and hidden menace.

Based on Oscar Wilde's 1890 Victorian gothic novel, the film, directed by Oliver Parker, has received mixed reviews in Britain, but Firth's performance has been universally praised.

"Henry doesn't get his own hands dirty, he gets off on seeing a younger man destroy himself. Oscar Wilde never elaborated on his motives, but it is always more fun to be a bad person. Every actor wants to get their hands on a villain really, particularly one with all the best lines, a witty villain. It is not something that comes my way very often.

It is not really me, I have to say, it is usually Hugh Grant or Rupert Everett who gets to do louche. It can make you very neurotic when you are trying to play nice people on screen."

(Everett remembers meeting a young Firth on the set of Another Country: "He played guitar and was very left-wing; he was going to give all his money to charity. I fancied him until he got the guitar out.")

Vanity, mortality and ageing, Wilde's prescient themes, are of course still an obsession, especially for an actor whose looks are magnified on the screen. Looking at it now, the wet shirt scene in Pride and Prejudice was rather tame, but within the repressed sexuality of Jane Austen's world it heaved. You get a sense Firth is relieved to have outgrown all the tedious heart-throb stuff; he was never comfortable with it and often said it had nothing to do with him.

"Since the beginning of time there has been a fixation on youth and beauty, but all the things gravity has done to me I have earned. I get a terrible shock every time I see myself on film.

"I don't think there is a single actor alive who enjoys watching themselves. It is all hideous, really. There are always certain mannerisms and gestures that you imagine you don't have but you do have. It is just better not to watch yourself."

As Dorian Gray does not age because of the painting hidden away, Firth in the film becomes an old man beside him. "When you take the make-up off at the end of the day you think, there is that wrinkle gone and there is that wrinkle gone, and you are scrubbing away and this one is not going anywhere. Oh dear, that is just me."

But running in tandem with all the frivolity, silliness and comedic turns is another Firth: there is a deeply serious side to him that is not often seen on the screen. He is an intensely political man who is heavily involved - often behind the scenes - in a wide range of humanitarian projects and charities, including Amnesty International, Make Poverty History, Fair Trade and Survival International: "They deal with trade, climate, arms trade, problem of child soldiers, human trafficking, a lot of problems with poverty," he says.

He has received many more awards as a defender of poor countries than he has as an actor, but speaks quickly and resignedly about these things as if he does not want to call attention to his herculean efforts on behalf of the downtrodden.

He is director of Progresso, a chain of coffee shops in London where the profits are directly shared with the primary producers in places such as Ethiopia. And here he does get his hands dirty, goes to Africa, sees it for himself. "I represent Oxfam on a number of things like Fair Trade, and as a global ambassador I was over here (Italy) boring everybody rigid for about six months in the lead-up to G8, just trying to mobilise issues."

"We are a political family," his wife, Livia Giuggioli, said in Rome in 2007, when I interviewed the couple for the documentary they had produced, In Prison My Whole Life, about the injustices to political activist and former Black Panther Mumia Abu-Jamal.

"We wake up talking about politics. Colin is one of those people who researches everything and gets obsessed. He will wake up in the night and want to talk about the war in Afghanistan. We're a great match because I am the ballbreaker and he is the brains."

Giuggioli, a former film producer, and her brother run an Eco Shop in London. She is a very thin, stylish, chiselled, long-legged Italian beauty, confident, loud, vivacious, energetic and as committed to humanitarian work as Firth. They have two small sons and he has an older son from his relationship with actress Meg Tilley.

Giuggioli's Roman family was not overly keen on Firth at first. "They are an Italian family with all kinds of prejudices," he once admitted. "She had been dating a doctor who was far more suitable. As an actor I had two strikes against me; I had a kid, that was three strikes. I didn't speak the language, I was 10 years older. It was like two tribes meeting. I had to court her." The two have been married 10 years and live between Umbria and London; accepting his Venice best-actor award, Firth spoke fluent Italian.

Firth comes from an academic family. His father is a history lecturer at Winchester University College and his mother is a comparative religions teacher. Three of his grandparents were Methodist missionaries and he spent his early childhood in Nigeria. There is clearly a genetic blueprint for missionary zeal that has manifested itself in him.

Had he not walked into the family dining room when he was 14 and heard the words "I am going to be an actor" come from nowhere out of his mouth, he would almost certainly been an academic or aid worker.

"That work interests me and it is not just that it interests me. I hate some of it, actually. But I don't have any choice in the matter. When you have talked yourself into a position and feel a certain way about things, you can't unthink them or go back. Also, you develop relationships along the way which keep you going. There are people who begin to depend on you for certain things.

"It becomes a part of what you have committed to and is completely different to acting. Sometimes it is absolutely hideous. A lot of stuff, the petitions, the campaign work, the lobbying, it feels futile sometimes and also I realise that a lot of people don't want to hear these things from actors, so you are constantly dealing with the fact that however passionately you feel about something, you are not the one with the authority to tell anybody. You are not a sociologist or a politician or a journalist, people are inclined to say stick to your knitting, you know."

In a way he has stuck to it. Firth is much as Everett remembers him from the Another Country days in 1984: still left-wing, strumming his metaphorical guitar as he traverses the wide divide between baked African plains and the indulged life of a movie star.

"It is bizarre, really. A film set is a fairly strange, kind of alternative society, really. It doesn't reflect the outside world at all. I think actors are essentially juvenile. There is a retarding element to the job."

Genova opens on November 5. Dorian Gray opens on November 12. A Single Man opens on February 25, 2010.