As with any enduring couple, there are times when David Suchet grows a little frustrated by Hercule Poirot. "It's his detail," he sighs. "Well, there's no question he's obsessive-compulsive. Really so. When I'm him, every time I wash my hands out comes the lavender eau de toilette." He rubs his hands together in a mime of hand-washing. "It's not a precious thing - you often hear writers say their characters take over, and Poirot takes over. And that chap can be really irritating."

They have been together 20 years now, Suchet and Poirot, a union that began in 1989 with The Adventure of the Clapham Cook, and has now reached a glorious anniversary box set containing all the episodes to date. After all this time, people are occasionally surprised, he says, to find he is very little like the Belgian detective. "I'm not Poirot, I'm not 22st; it's not just the moustache, it's the whole persona." In person, Suchet is much warmer than Agatha Christie's character. Today he is dressed dapperly, with precisely the kind of immaculate air that you rather imagine Poirot might appreciate, but he smells not of lavender water but of a rich, woody cologne, and when he speaks it is deeply, with the kind of voice that seems to begin at the knees and rise up.

Suchet was far from unknown by the time he accepted the role of Poirot. A seasoned stage actor who had spent 13 years with the Royal Shakespeare Company, he had also led two successful television series: Freud, in 1984, and Blott on the Landscape the following year. "But as well-known as I was, it was Poirot that shot me into most households," he concedes.

Before Suchet, Poirot had been played by numerous actors, most memorably Peter Ustinov, who brought to the uptight little man a flamboyance and theatricality. Suchet's main concern about filling Ustinov's shoes was that his interpretation would not be entertaining enough. "I love Peter Ustinov and I think he was a great Poirot, but he was not the Poirot that Agatha Christie put in the book," he says. "And because I'm a character actor I always work in company with my writer, be it Shakespeare or anybody else. I try and work out why they wrote my character. With Poirot, when I started reading the books, I met somebody that I had never met. Though I had seen the role on screen many times, with many actors, I'd never seen him. And I thought, that's all I can do, because that's me. I will work to what's in the creator's mind, and to every particular detail did I go to create the Poirot that Agatha Christie wrote."

It is this particularity of detail that characterises Suchet. He has made an art out of becoming Poirot. "It's a very deep process, a very complicated process. I suppose I could be accused of taking acting too seriously and losing the fun of it. I do take my work very seriously, I take on the responsibility of it. So with Poirot, because he is so different from me, mainly in the way he thinks, I can't just come in sleepy, bleary-eyed, sit in the makeup room, learn the lines, have a cup of coffee, read the paper, make a couple of phone calls, dash on set, do the performance and then go home." He pauses. "I can't do that with that chap."

The process begins when he is collected and driven to the set. He sits next to the driver and begins to read his dialogue for the day ahead. "And all of the dialogue for the next 10 days," he adds. "So I'm always working 10 days ahead. Because the man, that little man, he's a walking brain really. He doesn't care about his own body, but he does care about this," Suchet taps his head, "that encloses a brain, his little grey cells; that's how he lives and how he works. And so his dialogue has to come from the back." He touches the back of his head.

"It can't be like I'm speaking to you now, totally in the moment, because as you're speaking with him, he's analysing you, and will lock on you - there will be no one else in the room, and he will speak his assessments. Everything is from behind here," he whirls his hand at the back of his skull, "and everything is fiery here. So his dialogue has to be so well-learned by me the actor so I can then just put it out like ..." his voice grows pitter-pattery, "and it comes out quite incisive."

He can't eat much on the days he is Poirot "because that padding is not the most comfortable, especially in the summer", so he has a bowl of fruit and heads into makeup. "And then as I'm being made up I'm thinking about the day," he says. "And I'm watching, very closely, the face change. And it does change, very subtly. My makeup artist and I work very closely together, so every detail is done and the hair is all put back. And then the touchstone, absolutely the pivotal point - the moustache - goes on. And as soon as my lip feels that moustache, two things happen - first of all, I know he's there, but it also gives my top lip a very, very slight restraint. So I can't smile like that," he grins broadly, "I can only smile like that," he gives a tight half-smile. "And 20 years of that, I don't know what it is, but psychologically it enables me to come back to him. And from then on, until lunch, I'm him. Soon as the moustache comes off, it's OK, I'm fine."

There have been other detectives in Suchet's life: "A very, very tough policeman in [the BBC's] NCS Manhunt," he remembers quite fondly. "I was the head of plain-clothes police. Very rough, tough. I love those characters. Nothing like Poirot." And much earlier, there was Inspector Japp in the Poirot mystery Thirteen at Dinner, when Ustinov was still in the lead role. "It was the worst performance I've ever given on film," Suchet says firmly. "I really didn't know what to do as the character. And I said to Ustinov, 'Peter I don't know what to do with Inspector Japp.' And he said, 'Well, play a policeman!' And I didn't know how to play a policeman." He read the book over and over, and found no inspiration. "And in the end I ate in every scene and put on a cockney accent. And I was terrible. But thank God, because they were doing a series of films, and they were looking for a permanent Inspector Japp. And if I had done more as Japp I would never, ever have been offered Poirot later on."

It was unusual for Suchet to flounder in this way; his career has been built upon his - remarkable - ability to inhabit a character. "I'm really not interested in showing me, or playing me. My gift as an actor, given to me, is to be able to become other people."

At times this has required him to become contentious public figures, such as Robert Maxwell, or Sigmund Freud. But you sense he is drawn to the controversy, to the layers to get under. "Yes," he says, "it's getting behind the skin to justify any behaviour. When you're playing Maxwell, you can't play a bad man, you have to play a man that you believe; you have to be deeply, deeply convinced that he is right." Freud, he believes, was the biggest challenge of his career. "It was because of what I had to do to become him over six hours of television: the research, taking myself off to Vienna, studying with the Freudian analytical society and just getting into that whole area." But with any of his roles, getting behind the skin is a process that begins with fascination, with a delight for watching and listening. "I'm never bored, never ever bored," he says. "If I've got a day off I'll sit in a cafe and watch and observe. I'm a great observer."

Born in London in 1946, Suchet was sent to boarding school in Somerset, along with his two brothers (one of whom is the newsreader John Suchet). It was here that he discovered a passion for acting. This led him to join the National Youth Theatre when he was 18, and then to study at the London Academy of Music and Dramatic Art. He had, at this stage, yet to find his niche. "I didn't know what I was going to do," he recalls. "I was nearly chucked out of drama school at one point; I don't think anyone had much faith in me, early on. I didn't really fit in." Things changed when Suchet found himself cast as Mayor Hebble Tyson in the Lady's Not for Burning. "I thought, 'I've been cast as an old mayor.'" Suchet looks perplexed. "And I really, really worked on him and started changing myself, I started thinking how he would age, and I got such a buzz from it. It was the whole process of becoming that person. And that got me the student drama prize of my year."

His place as a character actor was only reinforced when, fresh out of drama school, he began playing in repertory theatre. "However much I may I have romantically imagined that there was a leading man in me, it was soon destroyed," he remembers. "I have a fairly low voice, and a fairly stocky shape, and in rep, you're immediately the middle-aged man, or the mayor, or a businessman, or a crook ..."

He is wary of always being typecast, though keen to stress that his association with the role of Poirot has never constrained him. "Actually, where I was likely to be typecast for anything, was playing Arab terrorists in American movies," he says. "And as a consequence, I won't do any more. I'll play baddies. I mean look at me, I'm not going to play juvenile romantic leads," Suchet laughs heartily. "Those days were never there ... well," the eyes twinkle, "there may be a little lonely granny out there we could make a movie about." He has to date played three terrorists. "But there was a time when everything that was coming in was terrorists, Arab or Israeli or Middle Eastern."

There is perhaps just a wisp of regret when he talks about film. "I would have liked to do more big movies," he admits. "And the reason I say that is not because I want to be a star, but what I would have liked to have done is reached a different audience with my work."

He is convinced that he will never make it in Hollywood now. "But I may do it, still - there is a hope, not a frustration - in European independent films, and I would reach a different audience that way." A decade or so ago, he saw the glimmer of possibility when he appeared in an American independent film called Sunday. "I played an American who'd been downsized, and he was living in a dosshouse, and he actually falls in love and it's a lovely story." It won the Sundance film festival and earned Suchet a nomination for best actor, but still, very few people have seen it. "Every European country bought it," he recalls, a touch ruefully, "I think it was bought in America, but it was never bought in England and it didn't run." He summons an air of nonchalance. "Weeeelll," he says, "there's no business like showbusiness. I think it's good for any actor to be happy. And not to spend one's life hoping for something else. I think it's a message for life, really: just be happy where you are. And I've always been happy where I am."

He is content these days, married for more than 30 years to the former actor Sheila Ferris, with two children. He is enjoying, too, a time in his life when he has "the chance to give back" - he has become an active participant in the Canal Trust, a nod to the time when he and Ferris, then both impoverished young actors, spent the early years of their marriage living on a narrowboat.

There are 10 Poirot stories left now, four of which will be completed this year, including Murder on the Orient Express. "Which is the big one," he smiles. "Then there will be six to go." It is not, he adds, a forgone conclusion that he will finish the entire series, "especially in this economic climate. I'm very lucky to get to do four more." But he is hopeful that he will get to do them all. "I'm not bored," he says, "not bored at all. He's irritating, but he's wonderful as well, and he's so interesting to be. And I'm going to eat them up"

• The Poirot box set is now available, priced £159.99