The last time I visited the Portobello Hotel was to interview gigolos as they were photographed, oiled-up and naked, in a bath. I hesitate to mention this to Colin Firth since our conversation in the hotel’s breakfast room has hitherto had an earnest, thoughtful air. But he seizes on the subject: “Were they good-looking? Did they take pride in their work? Were they gay? Who were their clients?” They love a bit of filth, do actors, and Firth breaks momentarily with his sensible interview persona to tell me that Rupert Everett, a rent boy before drama school, had a client who, many years later, became a close friend (although Everett has never revealed to him they’d met before in seedier circumstances).

But then, are our romantic leading men so far removed from gigolos? Both make a living knowing how to arouse and satisfy female desire. The differences being that film stars only pretend and are much better paid. Firth is in the highest echelon of British male fantasy lovers: if Clive Owen is bit-of-rough/soft, Sean Bean bit-of-rough/hard and Hugh Grant posh/light, Firth is probably posh/dark. His appeal is not in what he expresses but in what he is struggling to contain - Mr Darcy having to douse his ardour with cold water or the artist Vermeer in Girl with a Pearl Earring, channelling a longing for his housemaid-model into fervent brushwork.

Firth serves a well-read female clientele who abhor the obvious and crude, who could only fancy a mainstream Hollywood hunk in an ironic way. He is subtle, complex and refined enough for a number of unlikely ladies to emit earthy “phwoars!” when I mention his name, and grill me jealously about what he was like.

So, for them: Colin Firth this morning is wearing an unstructured midnight-blue velvet jacket over jeans. He is as tall (6ft 1in) and as broad as a leading man should be but seldom is. He is made of excellent stuff that is ageing well: lush, wavy hair, good but not Los Angeles-white teeth, the jawline of a leading man but the eyes of a watchful boy. The smile, a surprise since it is rarely part of his on-screen armoury, renders him younger. The sexiest thing about him, I find, is his voice: rich and warm, with a nervous edge that in films plays as choked passion. So later, when guests keep blundering into the breakfast room and he says, “Come on, let’s find a room downstairs,” an involuntary exclamation mark pops up in my brain.

He speaks in well-formed sentences with a very precise vocabulary. He does more interviews than most equivalent stars, and I suspect he likes to use them to demonstrate his intelligence. Because, at his age, 43, he seems to realise that being a romantic lead, which was far from his career goal, is not enough. He is queasy about his sex symbol status, even queasy about acting itself.

His latest film, Trauma, a low-budget British chiller by Marc Evans, director of cult horror flick My Little Eye, is a return to grittier, quirkier pre-Darcy roles. Firth plays Ben, an artist who lives with his ant farm in a creepy converted hospital in Hackney, East London. We meet him waking from a coma after the car crash that killed his wife. But Ben is mentally unstable, an unreliable narrator, so Firth, who is in every scene, does a lot of wild-eyed unshaven emoting as Ben’s sanity unravels. A gruelling part, I suggest.

“Oh, we love all that,” Firth says, sardonically (“we” being actors). “You can feel a bit of a char-monkey sometimes just showing up, getting your make-up on and phoning something in, then going home. In Trauma I was never off the set, which means, after a while, you are part of the decision-making process.”

Later, puzzling over the phrase “char-monkey”, I do a search and the only thing that pops up is an interview with Hugh Grant: “Imagine what it’s like, at 42, to be sitting in hair and make-up,” says Grant. “It’s ridiculous. It’s all right if you’ve written the film. But to be wheeled on, a char-monkey, at the age of 42. I hate it.” Firth gives me an only slightly more measured tirade: “It’s a fairly infantile day - you’re given a time to wake up, you’re driven to work, someone puts on your clothes. You’re treated like an 18-month-old child!”

The romantic lead’s midlife crisis appears to manifest itself as professional self-disgust. Being adored for your handsome plumage starts to feel empty, and not a little sad, after 40. You crave manly substance. But how to acquire it? Grant tried producing movies with his company, Simian Films, but that has stalled after two non-hits, Extreme Measures and Mickey Blue Eyes. Firth, however, has no ambitions to produce or direct, and despite contributing a short story to a collection, Speaking with the Angel, edited by Nick Hornby, he says he lacks the self-discipline to write.

“I’ve been deformed by the rhythms that acting gives you, which make you fickle,” he says. “You give yourself to a job for three months as if nothing else in the world exists. And then you drop it like a stone. As a director, you have to make that investment over a couple of years; I don’t know if I’ve got it in me.

“The trouble is most of us choose what we want to do when we are very young, and if it goes well your success can tie you to it. I sometimes feel I’m stuck in a profession only a 14-year-old would choose. Even though I love it.”

Fourteen is the age at which Firth, who plodded academically, devoted himself to acting. After school he found backstage work at the Shaw and National Theatres before drama school, and was then instantly scooped into the stage play Another Country with Rupert Everett. The film version followed. Firth has never done the struggling actor bit, toiled in rep or been out of work.

The Firth template was moulded early on: the sensitive victim of an experience so terrible it shuts down his emotions. In Tumbledown and A Month in the Country the damage was done in battle, in the Falklands and First World War trenches respectively. It was this ability to convey simmering internal conflict - as much as Firth’s dark looks - that made him a perfect Mr Darcy. Any actor who could embody every teenage girl’s literary crush and that most potent archetype of female desire, the distant, withholding but powerful man unlocked by love, would stir half the nation’s loins. But Firth says it was “the most improbable thing ever to happen to me as an actor. People would have howled with laughter if I’d tried to predict it. In fact, they did when it first happened.”

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