Nigel Hawthorne sits in the lobby of the Queen's Theatre having his picture taken while passers-by on Shaftesbury Avenue crane and stare. Of course. TV's Sir Humphrey —they nudge each other and smile. He looks mortified. The night before, at the preview for his new play, the restoration comedy The Clandestine Marriage, he had stepped forth from the curtain call to appeal for donations to the Terrence Higgins Trust, and then rattled a bucket down in the stalls with that same pained and awkward expression. Even while fans were saying how they had died laughing at his vain and deluded Lord Ogleby, he clearly wished them far away. He has said it often enough: he hates taking his bow, publicity, scrutiny, talking about himself. 'When I act I'm in my own little world, and when I leave it I'm not like an actor at all."

He is, rather, a gardener, a walker, a keeper of dogs, a countryman who proudly shows you his new oiled green jacket and sighs with relief each night as he catches the train back to his farmhouse in Hertfordshire. This determined distance from what he calls, with a shudder, "the showbiz side of things", is in part a natural reticence; he claims to be so nervous that he shook before early recordings of Yes, Minister and popped beta-blockers. It is also the result of coming late to professional success — he was in his forties for his first leading theatrical roles, in his fifties when a sitcom thrust him into celebrity, too old to enjoy the swanning and basking that a younger 'discovery" might have liked. For if young fame is damaging, its late equivalent is a thing to be treated with suspicion. It has been absent for so long that it might choose to disappear again at any moment. Unsurprising, then, that the softly spoken 65-year-old talks a

lot about being grateful. Besides, Hawthorne has been to Hollywood and hated it, was appalled by the warring vanities of Sylvester Stallone and Wesley Snipes on the multi-million-dollar action thriller Demolition Man, in which he played a villain in Japanese robes. "It wasn't very me," he says with perfect understatement. It should not therefore surprise us, that when The Madness Of King George, the film of Alan Bennett's play The Madness Of George III, opens in New York later this month, its star will still be happily engaged in a far nobler pursuit, in a play first performed in 1776, and a part written by David Garrick for himself — though he never played it. "It's not really my scene, openings and all that, I don't like all the baloney. I like staying hidden... it's a silly thing."

As the first play he has directed in the West End, Hawthorne has made of The Clandestine Marriage — a comedy of manners, matrimony and dubious morals — a great screaming camp romp, resplendently costumed, minimalistically designed. teetering on the verge of seasonal pantomime. For Hawthorne it was a perfect vehicle, an ensemble piece with no starring parts; it was also one of the first plays he saw when he arrived from South Africa in 1951, hellbent on an acting career in the city of his waking dreams. He had £12, a baggage of parental disapproval and an unhelpful accent. "When they saw this strange, sunburned, accented young creature, they must have wondered what they could possibly cast him as - so they didn't."

Up and down St Martin's Lane he trudged, begging bit parts from agents in poky offices, understudying and assistant stage managing. These were the days of the great autocrat directors such as Tyrone Guthrie and Glen Byam Shaw, when the whole of Shaftesbury Avenue was controlled by H M Tennants who, as a sop to the profession, held open auditions for cap-in-hand hopefuls such as young Nigel "We did our auditions and a lady called Daphne Rye would say 'thank you' halfway through, and then we'd all go out into the weather again." It went on for years and had him thoroughly dejected.

He is not against early struggles on principle, however, thinking it a mistake for young actors ever to turn down work. He demands discipline and line-learning from his own team, and is amazed by the letters he receives from aspiring actors asking not so much for advice as for hard cash (which, if amused or moved, he sometimes dispatches). Though there are limits to one's endurance. Cold and defeated, Hawthorne headed back home to Cape Town, where at least he worked. "But if I was to be judged as an actor, I had to be judged among the best, and I knew the best were English." So he returned, with a little more experience but not a lot more luck. He found a mentor in Joan Littlewood, whose Theatre Workshop he joined in 1965, and gained an agent with his Falstaff in Sheffield, but he was not among the Bates and Finneys and Courtenays, all marking the 1960s generation as their own.

Eventually, five series of Yes, Minister turned him into a household name, providing a platform for his versatility, his verbal fluency, his deadpan, his comedy - from now on he would pick and choose his roles. In the audience of the first recording was the writer Jack Rosenthal, who took him aside and asked if be might send him a script of The Knowledge, in which Hawthorne was to play the cabbies' scratchy examiner, followed by the parsimonious grouch in The Chain. "Like all good duos," he says of his first big break. "it was a male-female relationship, the henpecked husband, George and Mildred, really." Ask him if he minds remaining forever Sir Humphrey and he looks surprised. "Not at all." It won him a CBE, after all, got him invited to Downing Street with Paul Eddington and even, as if he were a real White hall mandarin, to an official reception for a visiting dignitary; which he typically declined.

He has become an expert at saying no. When Trevor Nunn got around to a Stratford invitation in the late 1970s, he turned it down. Ten years too late, he replied. He had knocked on the RSC's door for too long with nobody bothering to listen. Oh please, they said, he could do Malvolio and Cassius and Cardinal Wolsey in Henry VIII, but Hawthorne stuck with his life of ordered independence. "I never wanted to get into the trap of waiting for my next big role... I have been asked to do Lear and I know Ian Holm has also been asked. I hope he does it because he would do it much better than me. I don't think I have the vocal prowess, I'm just not that sort of an actor."

Besides, as the mad-monarch roles go, few could better Hawthorne's George III, an echo at times of the classic role, a man beset not only with dementia but with the plotting and vying of doctors, court and royal clan. The film was shot last summer at Shepperton Studios and on location at Arundel Castle, Windsor Great Park and Painted Gallery at Greenwich. It was, he says, great fun, if a little risky at times, especially as he galloped across country in costume and sword but no hard hat. "In one still, my horse is airborne, with all its feet off the ground!" But, for Hawthorne, getting the film made was the important issue: a triumph of substance over style, of talent over big names, of British seriousness over the jive-talking money machine of American film-making. All the supposedly difficult actors (Mirren, Holm, Everett, Donohoe all star) were pussycats because they enjoyed it so much, while members of the various stage casts played bit parts just to be involved.

Best of all perhaps was his friendship with Helen Mirren, playing Queen Charlotte, whom he had never met but soon came to value as an adviser. "I am a bit of a brooder and I'd start regretting things I'd done the day before, but she would tell me not to look back, to learn to let it go." Both Hawthorne and Nicholas Hytner, the director, were employed by absolute demand of the writer, Alan Bennett, who refused to let his script be filmed without his leading actor. This loyalty, much more than his stint with Sly and Snipes (done to help his chances in The Madness Of King George), secured his first big movie part; it also avoided the disappointment he felt when his lead in the 1978 production of Privates On Parade went in the film to John Cleese; and again when his definitive C S Lewis in Shadowlands, a part that won him a Tony Award in 1991, went to Anthony Hopkins. Though he promises he did not mind this, we can only reflect that he has not seen, and probably will never see, the movie version. "I just choose not to because the part is too personal for me... . But I always knew it would be impossible for me to do it, they needed a big name." What, then, is so different about George III? "Only Alan Bennett I suppose ... And this one will sell on merit alone."

On stage the role of the stricken academic Lewis took two years of his life. The deranged monarch stole nearly three. No wonder The Clandestine Marriage, with its three-month London run and inconsequentiality, feels like a holiday. He no longer needs the afternoon naps on the single divan in his dressing room, the same one he used when he was filling the house with teary ovations in Shadowlands. 'Such extreme situations are cathartic to act," he says. "Somebody said to me in New York that if I went to a psychiatrist I'd pay several thousand dollars for the therapy I was getting every night in that part. Such a role teaches you something. If you can release emotion you are privileged, and if you can do it in public rather than having a good old blub in the loo, you feel released — and you know you are releasing hundreds of people in the audience.'' He has two thick files of letters from bereaved spouses who were advised against seeing the play but found it uplifting. And another from sufferers of the king's malady, porphyria, thanking him for airing the problem. "If there's one thing the theatre does other than entertain or be sentimental, it is to help people through a crisis."

The movie-star-in-waiting splutters at the idea of imminent Oscars and global adoration. He is just grateful they did not give his George to Sly. Grateful, that word again. "I am grateful that things have changed," he says of his late success. "But I don't feel resentment for what has happened. There's no place in my life for it. It's like Helen says, you have to let go."