When a man admits to being a liar, can you believe him? Michael Gambon says the best piece of advice he was ever given was to tell the truth, but obviously that only applies to his on-stage existence: off stage the man is an inveterate and self-confessed liar who takes an often unbridled glee in leading journalists up the garden path.

He doesn't especially enjoy the interview process, so he turns it into a kind of a game, slipping in some outrageous whopper about his past life to see if the interviewer is alert enough to notice or brave enough to call him on it. A favourite yarn is being forced to give something up for some unfortunate reason. Being gay made his eyes water so he had to pack it in. Similarly, his promising career with the Royal Ballet ended when he fell off the stage and through the timpani. None of it is true. But all of it is outrageously entertaining.

We are in the library of the National Liberal Club in Whitehall, London: a suitably fusty setting given his appearance in the latest Harry Potter film. After a career laden with awards for stage work, this, ironically, is the role likely to gain him the international acclaim he so richly deserves - but more of that later.

Outside there is chaos, as a bus with a leaky fuel tank has brought a chunk of London to a standstill. Gambon had to abandon his official car and walk from Trafalgar Square - but fears that the delay might have blackened his humour are quickly dispelled. ''One tiny patch of diesel on the road and Ken Livingstone shuts down the whole place,'' he snorts as he ambles in, but otherwise his mood is good. Dressed in an expensive-looking tan linen suit and an even more expensive-looking blue cotton shirt, he settles himself quickly with a cheery hello.

A great bear of a man, Gambon has the expression of a slightly distracted favourite uncle. Yet he is constantly interested in what is going on around him: he is amused, for example, by the Harry Potter props that have been dotted around the room. Although he will be 64 this year, he still retains boyish enthusiasms. He has an almost childlike sense of wonder and mischief and is, by his own admission, an unreliable witness to his own life. Still, after ten minutes of chatting he seems to have been scrupulously truthful, and I confess to feeling a little slighted. He is both dismissive and reassuring. ''I never like speaking about myself, so I am constantly telling lies. But I can't tell you any new lies because this is all pretty basic factual stuff.''

Now that I've tipped my hand, however, Gambon is amused that I know he may lie. His enthusiasm is whetted, and trying to extract information soon becomes like picking up mercury with a fork. He might be married; then again, he might not. He might live in London; again, possibly not. ''Greater London'' is his description, but that covers a lot of ground. Is it, then, a notion of letting daylight in on the magic by providing too much information? Is he a man who exists only in performance? ''I suppose so,'' he says, sighing. ''I suppose it gets in the way if you walk on the stage and the audience is saying, 'Oh, there's that man who collects toy trains.'''

Remarkably, among the stories of eye-watering homosexuality and stumbling balletic ineptitude, there is one story that shines like a beacon. You would bet folding money this one was a lie, but it turns out to be true. Michael Gambon once auditioned for the role of James Bond. ''I did, yes,'' he insists, catching my look of surprise, ''along with a lot of other people. There were about 20 of us. We walked in and Cubby Broccoli said, 'I've been stung by a model.''' The producer's withering description of George Lazenby is rather unkind - but they did need a new Bond, and fast. The part eventually went to Roger Moore.

The mind boggles as to what a Bond film with Michael Gambon might have been like. More Le Carre than Fleming, Gambon's Bond might have been one of his rich vein of characters who have been dealt seconds in the great game of life - and are all the more interesting for it.

Those signature characters - epitomised by the disfigured writer Philip Marlow in the 1986 television series The Singing Detective - might be a product of his upbringing. He was born in Dublin but his family moved to London when he was very young and he lived the life of an immigrant in straitened circumstances. Schooldays were miserable. ''I have no happy memories of school whatsoever, just of being hit,'' he says, with the first hint of bitterness. ''I had one master who used to spend his time just digging me in the chest with his fingers.'' He pokes the air for emphasis.

Hardly surprising, then, that Gambon left school when he could and took up an apprenticeship as a toolmaker at the local Vickers works. He served his time, but his adult passion for things mechanical - he and his Harry Potter co-star Robbie Coltrane spent hours on set talking about cars - is probably the only legacy. He always wanted to act. ''It's like being a priest or a doctor or a nurse: you have a calling. That sounds a bit grand, doesn't it?'' he says, chuckling in embarrassment. ''But it's a similar feeling, I suppose, because you never think of the consequences.''

So how did he get from the Vickers shop floor to being dubbed the Great Gambon by Sir Ralph Richardson? After all, he didn't see a play till he was 19 and had no family history of performing. In fact, the closest he'd got to acting was being mad keen on the pictures.

''I wrote a lot of letters. I just felt the need,'' he says. ''I went to see James Dean in Rebel Without a Cause, and when we came out I was walking down the street with my friend and we passed a theatre where there were display cases of photographs of the actors. My friend said to me, 'They're doing what he's just been doing,' and I hadn't put that together, the theatre and the film. I thought, 'Well, I can do that.' You wouldn't think you could do that watching James Dean, but you'd think you could do that looking at a theatre box with two blokes sitting on a sofa. So I think that's it.'' He pauses. ''Does that make sense? It doesn't to me.''

Naturally, Gambon's acting career started with a lie. Having never acted before, the letters he sent out to theatre companies contained largely conflated CVs. One of them, however, found favour at the Gate Theatre in his native Dublin, and Gambon was taken on as a very junior member of the company. The Gate had been founded by Micheal MacLiammoir, the legendary Irish actor who was one of the last of the great actor-managers. A grand Dublin character, he had starred as Iago to Orson Welles's Othello.

''He was great,'' Gambon recalls with delight. ''He wore a wig in the street, a big heavy toupee which lifted at the front. He was always sweating and he never went out without make-up. I loved him. He wore a corset to keep his belly in and you could sometimes see the corset when his shirt was unbuttoned too far. He strolled round Dublin in silk shirts and lovely clothes, and I can't imagine Ireland in the early sixties putting up with him but they loved him. He would walk up and down O'Connell Street and Grafton Street dressed like that. We'd go out for a walk with him at lunchtime and he'd just smile and nod at everyone. He wasn't even Irish, he was from Cricklewood. He invented - talk about me being a liar - his whole life. He was born in Cricklewood and invented a life about being Irish and born in Cork. He learned to speak Erse and changed his name; his real name was Reginald

something. He was astonishing.''

For the record, MacLiammoir was born in Kilburn in North London and his real name was Alfred Willmore, but Gambon's point is well made. ''I only had a small part and he wouldn't even look at me when I walked on. We were doing Othello and he was Iago. I just remember how wonderful his voice was, that lovely rolling lilt he had. Then we toured all through Europe with it, one-night stands, and that's all I remember.''

It is easy to see how MacLiammoir would have shaped Gambon's idea of what an actor should be. The notion of this grand figure whose public persona was a complete confection and whose private life was closely guarded has plainly echoed down through the years. Even the rich, rumbling baritone that Gambon has made his own might well owe something to MacLiammoir.

Gambon was not long for the Gate, and in 1963 he found a place in Olivier's first National Theatre company. This launched a career in which he was for many years Britain's best-kept secret, known only to London theatregoers and the chattering classes. He formed a fruitful creative relationship with Alan Ayckbourn, who directed him in one of his greatest stage roles: as Eddie Carbone in A View From the Bridge at the National in 1987. This came the year after The Singing Detective, the role that made him a household name.

Yet Gambon had not gone into acting with a view to being well known. It hadn't even occurred to him that the two might go hand in hand. He concedes that these days they are virtually inseparable, but all he wanted to do was act. While he was building his reputation on stage he took what film work came his way: Peter Greenaway's The Cook, The Thief, His Wife and Her Lover, for instance. Since the beginning of this decade, though, all that has changed: in the last couple of years alone he has had roles in more than a dozen films. The biggest of them in terms of screen time was his turn as the villainous land owner in Kevin Costner's western Open Range; there was also a cameo opposite Gwyneth Paltrow in Sylvia, and before that a turn in the acclaimed period drama Gosford Park. Now, of course, there is Harry Potter and the Prisoner of Azkaban.

As an actor without a game plan, Gambon can make little sense of it. ''I'm quite surprised. Definitely surprised. Bemused, actually,'' he says, with the vague air of a man who has mislaid his car keys. He ponders the suggestion that he has suddenly become fashionable. ''It may be,'' he says, ''or maybe there's a shortage of guys of my age, I don't know. If you try and think of reasons I can't think of reasons, but maybe that's one. An English actor of my age, a stage actor I tend to play heavyweight parts. I think they might be a bit thin on the ground. That could be a reason.''

Then again, it could simply be that the film industry has finally cottoned on to what so many people have known for a long time: that Gambon is an acting titan, a national treasure.

None of his film roles occupies his time for very long - certainly not in terms of the time it takes to prepare a play - and he is apologetic about his lack of anecdotes, even about the Prisoner of Azkaban, the much-hyped third instalment of the JK Rowling adaptations. ''I've no idea, I was only on it for three weeks and I've forgotten it already,'' he chuckles. ''It was in the middle of last year and then you come to an event like this and I get a bit nervous because I can't remember any of it. It's another job, so I don't know what it means. I suppose I'll be more popular with children, will I?'' Suddenly it's an an Eeyore moment. ''There's something. You get lots of fan mail from kids, a big pile like that.'' His hands are about six inches apart. ''Hundreds and hundreds of photographs saying they want an autographed photograph of me as Dumbledore, so I sent them all off.

''I can't remember any of the films I've done,'' he continues cheerfully. ''You go from one to another and they all blend in to a big mass. You remember the costumes because you remember how you felt - that Western I did with Kevin Costner where I wore the big hat and the two guns, I remember that. And I remember this [Harry Potter] because of the costume. Richard [Harris] wore great big heavy robes, really heavy, but I wore just silk, just two layers of silk and carpet slippers, so it was the most comfortable job. The only problem is the wig and the make-up, which is quite time-consuming.''

Gambon plays Albus Dumbledore, Harry's headmaster, in the style of a roguish ageing hippy. He inherited the role from the late Richard Harris but has no qualms about playing a part created by someone else - ''I did it for years at Stratford'' - and has become tired of explaining it. He also plays Dumbledore as Irish. Once he got the hair and wig on, it seemed to bring out the Irish in him; a kind of homage, perhaps, to his fellow countryman Harris.

An intuitive actor rather than a technical one, Gambon has an emotional transparency that thrives in the theatre, where the performance is different every night. The strictures of screen acting were alien to him at first. ''I have strong theories about that, but then they get knocked apart,'' he says. ''I sometimes think the theatre is more demanding because it requires things you don't have in films, like it requires you to make the people in the front row believe you and not look an idiot to them while the people right at the back can hear you. So I think the theatre is different - and then I do a film and I think that's wrong because I'm doing the same in a film. I did a play recently where I spoke at nor-mal microphone level and they could still hear me. So my theories keep getting tipped over.''

Theatre gives him more of a kick than film, he says. The audience is right in front of you and provides instant feedback. He says he senses the minute he walks on stage whether it's any good because of the vibe coming off the audience. ''Also in the theatre you get four, five, six weeks' rehearsal. You rehearse like hell, you get through it, you bash it apart, you tear it apart, analyse it, turn it upside down, and by the time you do it in front of an audience it's like part of you. In a movie that doesn't happen because you don't rehearse it, you don't spend weeks with the other actors, none of those processes go on. You're pretty much out of the trailer and bang in front of the camera. I find that alarming. I always have.''

Alarming or not, Azkaban is the first of three Harry Potter films for which Gambon is contracted. Although he has a relatively short amount of screen time in this one, his role in the next is more demanding. ''It's 62 days over six months. Very nearly like working for a living,'' he says.

Gambon has earned his place in the spotlight but it is likely he will make little of it, just as he does with the knighthood he received in 1997. (Although Irish-born he has British citizenship, so is entitled to his gong.) Gambon doesn't use the title - ''I'd be embarrassed. It's a nice present they give you'' - and is a little surprised when I tell him that some actors are punctilious about their rank. ''They must be mad,'' he demurs. ''Tom Stoppard said the other day that turning it down is more arrogant than accepting it, which is quite truthful, so it suits me to say that.'' He smiles mischievously.

A stranger to the celebrity chat-show circuit, Gambon won himself millions of new fans with his recent appearance on Have I Got News for You. His air of confusion and bewilderment stole the show - yet, unsurprisingly, it was a lie. ''That was an act,'' he says, barely able to conceal his delight. ''I was speaking to the producer beforehand and said I was rather nervous, so he said to just pretend I didn't know what was happening. So I sat there and did that. It worked, didn't it? It worked quite well. And you can say what you like using that pretext.''

Gambon also reveals that Paul Merton has asked him to appear on Room 101, the show where celebrities get to consign their pet hates to an Orwellian fate. ''I know what I'll put in first,'' he says enthusiastically. ''Those 'Baby on board' signs in the backs of cars. Do you get annoyed by that? As though you would drive differently if they didn't have 'Baby on board' on the back of their people-carrier.''

While I can't disagree, I feel obliged to point out that these stickers actually started out as life-saving devices to alert the emergency services that there might be a child hidden in any wreckage. Gambon is halted momentarily mid-rant. ''Oh, I see,'' he concedes, before quickly gathering pace again. ''Yeah. 'We've got a baby. How dare you drive close to us?' What else? Four-wheel drives. Do you hate them?''

He's preaching to the choir again. What, I wonder, about journalists? Might they go down the chute in Room 101? Apparently not. He may tease them but he doesn't dislike them and counted the late, great writer and broadcaster James Cameron among his friends.

''There you are,'' he says triumphantly as he gets up to go. ''Not one lie.'' But, as he starts to move away, he turns back with a glint in his eye. ''Oh, I didn't tell you. I've got two broken ribs. I fell down in the bath.''

And the thing is, I really don't know whether or not to believe him. n

Harry Potter and the Prisoner of Azkaban is on general release.