Sir Ian McKellen is in his underpants, which is surprising in all sorts of ways. Since the room is not very large, one can't help clocking his various attributes ­ shapely blemish-free legs, a manly chest, sinewy arms. If there is a message implicit in his invitation to witness this costume-fitting for King Lear, the father of old-men roles, it is this: I may be playing the part of a man "crawling unburdened towards death" but wouldn't you say I was in good nick? To which the answer ­ particularly since McKellen is pushing 70 himself ­ has to be a resounding "yes".

He is relaxed, good-humoured, almost playful ­ helping himself to bites of the wardrobe-ladies' custard tarts, like a favoured child secure in the knowledge that his naughtiness will be indulged. "I want lots of medals," he pronounces. (In this forthcoming year-long Royal Shakespeare Company world tour, McKellen will also be playing another eccentric old man, fighting against fading, Sorin in The Seagull.) He's thinking of bringing in one of his own, which worries the costume chief, a former master tailor ­ "You don't want to do that, do you?" "Why not? It would amuse me," McKellen says. "It's only a knighthood ­ or perhaps I'll bring the CBE, which is a much more beautiful shape."

This McKellen could not be more different from the one I met 18 years ago. That interview took place not long after he had come out on Radio 3, two years before the knighthood, and not much seemed to amuse him then. He was prickly, defensive, refusing even to look me in the eye, and negative about almost everything. The overwhelming sense he projected was that he had been left on the shelf, personally as well as professionally. What made him really sore ­ forget the scores of awards and his undisputed mantle as the leading classical actor of his generation ­ was that he was not recognised in the street, unlike his theatrical confrères, the late Alan Bates and Anthony Hopkins, who had made the breakthrough into Hollywood. "It's a slight mystery as to why bigger film roles haven't come my way," he said plaintively, "but now I've been left behind."

Everything has changed in the intervening decades, and it doesn't seem too preposterous to suggest that McKellen owes his newfound sunniness to another quirky old man, the snowy-bearded wizard, Gandalf.

We bump into one another unexpectedly before the interview in a café close to the RSC rehearsal space. It's an interesting look that he's put together:Seventies dude, pimp from Shaft, meets Berkeley ageing hipster: tousled hair, spectacles, a long leather jacket (charity shop or possibly a very expensive designer), blue polo-neck (he's sufficiently self-conscious about what he refers to as "the nastiness" under his chin to have considered and rejected undergoing the knife ­ "I'd be too scared"), a medallion of some sort of green stone, artfully faded jeans with a brown velvet stripe on the outer legs.

Later, I ask if he's vain and he repeats back, "Vain? I dare say. What do you mean, vain?" When I comment on how stylishly he's dressed, for example, he says: "Oh, I see. Heh heh heh. Thank you. Well, I'm all done up for a photograph." The trousers, he says, are by John Varvatos, "A very nice designer for middle-aged men... or old men. I mean, do you think of me as an old person? I don't mind you saying 'yes'.

" He is all smiles and benevolent chuckles, endlessly amused and upbeat. I wonder, perhaps, whether he is in love, so rose-tinted is his view of the world, but he says the only man he has time for at the moment is King Lear. What I think is that he may be in love with life itself now that he is at last an international household name, and all questions seem to lead back to The Lord of the Rings. Is it strangely neutering becoming a national treasure, the people's favourite gay and all that?

"Hmmm. Heh heh heh. I'm going to take that as a compliment. However, when strangers come up to me they don't, on the whole, talk about gay issues ­ although some do. But usually it's, 'Oh, you played Gandalf' or 'You played Magneto'." (In another Hollywood blockbuster, the celluloid adaptation of the X-Men comics.) To which he can also add, presumably, his Sir Leigh Teabing in the Da Vinci Code.

"But national treasure? I'm not sure. I think most people treasure me for being GandalfŠ You know, what I like most about being famous is that after a lifetime of going into a room and almost sweating and wondering how long I can stand it and not enjoying parties ­ now if I go into a room with strangers, actually I'm not a stranger any more. It's a point of reference which means you don't have to talk about the weather and you can very quickly get off talking about yourself. It breaks the ice and that's what I like most about it.

"And it's nice being said hello to in the street as well, because if you lived in a village you'd be said hello to all day long, wouldn't you? But that doesn't happen in London, so it's very, very nice to feel I'm in a community."

Has he become grand, post-knighthood and mega-fame, I was wondering, but after traipsing through the grotty rubbish-strewn alleyways of Clapham High Street, dismal stairwells and up and up and up into a most unstarry room for our interview, the question seemed redundant. Working with the RSC is clearly a levelling antidote to the pampering excesses of Hollywood. Not that McKellen is in all that much danger of being seduced by the latter.

His partner for ten years, from 1978 to 1988, was the actor- director Sean Mathias, who got rid of McKellen's scooter and made it a mission to get him into decent clothes and generally smarten him up. "Sean couldn't understand why I didn't spend money on real champagne," he says. Cava? Prosecco? "One of those. It was just that I don't have a great taste for champagne, so one fizzy white wine is much like any other to me."

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