Sir Ian McKellen arrives at the Covent Garden Hotel looking very professorial with lots of papers under one arm and a harried air. It is mid-afternoon, and he wants some food - a bit of soup and plain pasta. 'Anything to drink?' his assistant Clair wonders. He looks at her, and then has a sudden thought. 'I'll drink the soup!'

He settles into a sofa. I tell him I have just been to my first civil partnership, and McKellen says he attended the union of Michael Cashman and Paul Cottingham. 'I was a witness with Michelle Collins. There was a groom's side and a groom's side, and you had to decide where to sit.' He says he got a bit weepy. 'They didn't call it marriage, although you can call it anything you want. The one thing you cannot mention is God, that is absolutely verboten. I suppose I'm a bit mean-spirited, but I really can't see why the government couldn't just say gay people can get married - that would have been true equality and so much simpler. But that hasn't been done because they couldn't face the furore. So they've passed a law that is not available to straight people - straight people cannot have a civil partnership, they have to get married - extraordinary.' McKellen has been in two eight-year relationships that he once referred to as quasi-marriages, but says he is 'very suspicious' of the institution. 'I can just sniff a divorce in the air. So the next thing to happen will be a gay couple getting divorced.'

The conversation moves to the great gay progress since McKellen took tea with John Major at Downing Street in the early-Nineties (about the time Stephen Fry affectionately renamed him Serena). Many fine advances in the legal framework, he observes, but social equality will still take time. He says he is in two minds about laws that curtail free speech, but suggests he would draw the line at homophobic religious leaders. 'On the whole I think it would be nice if people were polite to each other, although sometimes these leaders make it very difficult to be polite about Catholicism ...' Then another sudden thought. 'We're talking a lot of gay stuff - people don't want to read this.'

He is hoping they'd rather read about The Da Vinci Code and X-Men 3, both of which feature McKellen in prominent roles and open in a few weeks. But there are problems with this. When we meet there have been no advance screenings of either film, and I haven't read the book or comics on which they are based. 'The Da Vinci Code is the most popular book of our times,' McKellen muses, just in case I hadn't even heard of it. 'You've not read it? Well, you're at a disadvantage.'

'Or possibly an advantage.'

'Could well be,' he reasons. 'It's a very good part in a big movie. Very well paid. Filming in England, which is a curiosity. It's a rattling good yarn, and so is the film, or what I've seen of it. Very excitingly shot.'

McKellen plays Sir Leigh Teabing, the puppet master to Tom Hanks's hero. 'In the focus groups, the most popular scene is the one in which my character reveals to Tom Hanks and Audrey Tautou the theory that Christ fathered a child,' McKellen says, nervous he might have already said too much. 'Actually, I don't think anybody will say of the film, as they might with Lord of the Rings, "Oh seeing it will spoil my re-reading of it." I don't think many people will re-read The Da Vinci Code.'

McKellen will be 67 next month. He has never been out of work unless by choice, a feat he attributes to accepting many jobs other actors didn't fancy - 'one of the few advantages of being single and not having family responsibilities'. He has been working solidly for many months - Coronation Street, The Da Vinci Code, X-Men 3, Aladdin. When we meet he is nearing the end of The Cut, Mark Ravenhill's abstract play about imperialism and conscience, and after it he says he plans to take the rest of this year off. He says his priorities in choosing work are: is it something he would want to see? Who's directing it? Is it going to be difficult and therefore attractive? And then there are other things: 'If The Da Vinci Code had been filming in a place where it rains a lot I probably wouldn't have done it. Quite low down in the list is, "How much am I going to be paid?" I'd say I was quite cheap, but my main feeling about money is that I don't want to feel as though I'm being taken advantage of.

'Certainly I'm cheaper than Anthony Hopkins,' he continues. 'The other actors they asked to play Gandalf wouldn't go to New Zealand on that money for that length of time. I thought it would be a bit of an adventure. Tony Hopkins didn't think it would be an adventure. Tony is part of Hollywood. I'm an eccentric English actor, and there's a lot of us around.' He mentions Gambon, Jacobi, Hurt, Finney, Courtenay and Irons. 'People tend to think, "Oh, everything McKellen does is gold, he gets all the best parts now," but it's not the case. I'm one of a quite small pool of British fish swimming around, and we're netted occasionally. I made a film last year with William Hurt and Nick Nolte and Jessica Lange, and we can't get it released, no one wants to see it.'

I ask what it's called.

'Never Was.'

He says he is slightly worried that his face may become over-familiar. 'They've seen what I can do now - there's no more tricks. And I've turned down a very large number of highly paid small parts because I don't want to become a speciality act. I'll do those when I'm decrepit.'

Sheila Hancock has observed how, offstage, McKellen flops about 'like an overgrown puppy', and she probably wasn't just thinking of his large soft hands and foppish hair. Like all actors, he wants to be loved and his self-deprecation would probably extend to extremes ('I'm the worst actor in the world - really!') if it wasn't checked by an occasional 'surely not' and 'come come' from me. He says he has few ambitions to direct or produce, and that his one prolonged experience of making big things happen (the film of Richard III) left him exhausted. Accordingly, he has expressed no desire to run a big theatre company (the National being the most touted possibility) and no ambitions beyond recognition as a thoroughly dependable and engaging actor. Those who have seen him perform know he is something rather more than this. A few days after we meet, I see him in The Cut; goodness knows how the play would stand up without him.

His soup and pasta arrive, spaghetti with mussels, and he studiously begins to clank the shellfish on to another plate. He then has an engaging conversation with himself, about himself.

'Is Ian McKellen a star?' he asks.

'Well now, define your terms,' he answers.

'Is he an actor?'

'Yes, he's an actor.'

'Is he a classical actor?'

'Well, he's been in the classics, yes.'

'Is he a Shakespearean actor?'

'Well he acts in Shakespeare.'

'Is he Chekhovian?'

'He's done Chekhov as well.'

'Is he a stage actor?'

'Yeah, oh yes.'

'So he's not a film actor?'

'Oh, he's done films, too.'

'So what does he do, this guy?'

'He does what he wants to do, basically.'

'Fantastic,' I say, responding to his two-hander.

'Yes it is,' McKellen says, responding to the fact that he can do what he wants. He concludes, 'If I was a star, it would be difficult to go off and do Coronation Street. So I guess I'm not a star.'

I ask him if he is as excited about his work now as he used to be. 'No. I do feel if I had to stop tomorrow for some reason - health, or if I had a relationship which meant I couldn't work, then I think I could do it happily. I think 10 years more is probably all I've got, as someone who can nip on a plane and remember the lines and not fall over, so I'm very picky now. But I tell you, it's very very sweet at 6.30 in the evening when you're not working in the theatre. And not to get up early for a film... every part you play gives you grief. Everything's still a struggle. It's lovely not to have those things in your mind - deep worries, insecurities.'

He says he will devote some of his time off to writing, not least on his website. Here you will find a catalogue of his insecurities and achievements, some snapshots of McKellen on location, and a collection of his articles. There are thoughts on the coming out of Michael Barrymore ('The journey, not always as painful as you fear, will not be complete, however, until there is no one in the world, whom you know or whom you are to meet, to whom you would ever lie') and many notes from New Zealand and The Lord of the Rings ('Peter Jackson's team found the perfect location in the wide gravelly valley that leads down from the alpine range west of Canterbury...'). Particularly interesting is an article he wrote for the Bolton Evening News in September 1958 on the occasion of his departure from his grammar school. 'I long for a community where a faultlessly complete library is at hand, where a cinema declines to show bad films and a theatre presents new, good plays before they are old or have gone bad,' he wrote. 'Utopias must be dull places, full of satisfied people in a lifetime of retirement, content and as active as ruminating cows; perfection, like complete virtue, is fortunately inhuman. But when will a local cinema and a professional theatre give what I and so many others want?' His answer, of course, came from within, and we are the beneficiaries.

'Millions and million of people visit the website,' he tells me with a trace of bafflement. 'There was one day when I was the third most visited website in the world!' The day The Cut opened at the Donmar Warehouse? Or perhaps one of the Lord of the Rings films? 'There are more entertaining websites than mine,' he says. 'Full of jokes. Look at Alan Cumming's - that's beautifully done.'

But what you do get on McKellen's site is a memory of performances of great clarity, daring and humanity, and some foggy video clips of a few classic scenes. I mention his mesmerising performance in Trevor Nunn's RSC production of Macbeth in 1976.

'I'm able to judge if what I did was as good as you remember it to be,' he says.

'And?'

'Oh, I could give McKellen a few notes. I'm a better actor than I was then. And Edward II ... I think, "How did I get away with it?" I can't look at it, and at the time I thought I was the bee's knees. Upward inflections all the time! But I'm amazed at how good-looking I seem to be in some of the old photographs. Nowadays I would like more energy than I've got. Who wouldn't? And I don't like a sagging neckline. But apart from that I think it's all before me.'

I wonder whether the website is a prelude to an autobiography. 'No. Because you can't talk fairly about your relationships with people - it's only one side of the story. So unless you write a book praising every single person you've ever met...'

We meet not long after the Oscars, not long after Brokeback Mountain failed to win the Best Film award. McKellen couldn't have been less surprised. 'It's called homophobia,' he says. 'Nobody has ever looked to Hollywood for social advance. Hollywood is a dream factory. I love the way that conservatives think that Hollywood is a bed of radicalism - it couldn't be more staid and old-ladyship if it tried. The audience don't give a blind whatever about the sexuality of actors. Gay people fancy straight people and vice versa. It's all in the head, so what does it matter? You're not going to meet Heath Ledger. You're not going to find out... It's the image you're looking at and falling in love with. There will be girls who go and see those two unhappy gay cowboys and go home and have fantasy dreams about them. Lovely!'

McKellen was clearly upset when he failed to win an Oscar for Gods and Monsters in 1998; the fact remains that no openly gay actor has ever done so. Does that worry him?

A long think. 'Honestly, no. I'm not sure what to do about Hollywood, or whether I have a responsibility to do anything. I gave a talk to the gay and lesbian society at Disney, and friends of mine who are gay who work there didn't show up because they didn't want their employers to see them.

'And I think some other gay actors are frightened of me, because they think, "Oh if I do come out, I'll have to do what Ian McKellen does, and be a spokesperson." That's what Nigel Hawthorne felt. He said, "I don't want to be a spokesperson - leave me alone."'

After his Hollywood movies we will get a chance to see McKellen in what should be a monumental return to the classical stage, as early next year he reunites with Trevor Nunn at Stratford to play King Lear. He has been in the play before in less central roles, an experience that has made him both excited and cautious. 'When you're close to an actor playing Lear it can be very moving,' he says, 'because they're giving everything they've got. It takes you over. You get to see inside them. It's very exposing. I'm not looking forward to it at all.'

· The Da Vinci Code opens on 19 May; X-Men: The Last Stand opens on 25 May