Neil Pearson doesn't hold with talking about acting. It is, he says, 'a one way ticket to Pseuds' Corner in Private Eye', and that's one place he doesn't wish to visit. He is not interested in discussing the similarities between himself and the characters he plays, which, he says, rarely extend beyond the fact that 'they look like me'. That leaves only the celebrity fodder of his private life, the glittering leaf fall of relationships started and finished, of parties attended and houses bought. 'I do not like the idea of people who do not know me thinking they do,' he says simply.

Nevertheless he is here, sitting with a journalist at a restaurant table in Bristol, where he is currently based with a touring production of The Real Thing, Tom Stoppard's acid comedy about art and adultery. But it is because of an appearance next weekend, in an assured BBC television adaptation of William Boyd's novel Armadillo, that he has agreed to talk. Why, given his pronounced wariness of the press, is he willing to be interviewed at all?

He shrugs: 'The BBC was short of people to put forward.' James Frain, who takes the lead as insurance loss adjuster Lorimer Black, is relatively unknown. Stephen Rea, who plays Lorimer's boss, has historically been perceived by publicity departments as a tough proposition for press coverage because of his pro-Irish Republican views. That leaves Pearson to do the honours, even though his performance as a sweaty building contractor who may or may not have burnt down a hotel fills only a few minutes of screen time.

Armadillo was, Pearson says, a fun but compact gig, done as a favour for the director Howard Davies, whom he has known for a long time from his work in theatre. Listening to him explain this tricky and sometimes tiresome business of being a famous actor it suddenly sounds, I suggest, as if he is not at all convinced by the profession that has served him so well. He agrees. 'While I'm not about to announce my retirement in the pages of The Observer, the fact is that I'm getting to the stage where acting is what I do to earn money so as to finance the rest of my life,' he says, without wishing to discuss too much in detail what the rest of his life might be. 'Acting is not a vocation for me. I'm good at it. I'm probably very good at it, but I'm not the best.'

The problem is a marked shortage of good work. There are few substantial parts like the cynical, boozing journalist Dave Charnley in Drop the Dead Donkey, or the conflicted copper Tony Clarke in Between the Lines. 'There's always been a ghetto of excellence in television and I've been lucky enough to work inside it. Unfortunately it's shrinking and, not being a film name who can get movies made, I can't visualise what I'd be doing in 20 years' time. It's not a mid-life crisis. I'm just facing up to the reality. For me it's all down to what's in the script. That's why I stayed with six series of Drop the Dead Donkey and that's why I stayed with Between the Lines for so long.' But those good scripts are few and far between.

It sounds very dour and mournful, the desperate cry of an actor in his cups, but it's not. For a start he says he can't abide 'whinging actors. If I ruled the world they'd be the first ones to be put up against the wall and shot. Acting's a terrific way to earn a living.'

Pearson is good company, a sharp and witty conversationalist who keeps you on your toes. The writer Patrick Marber, who directed him in his play Closer a few years ago, and who now plays blackjack with him regularly in the casinos of London, describes a man who, off stage at the card table, knows exactly what he wants. 'He enjoys scowling at fools who take cards he deems ludicrous,' Marber says. 'He plays relentlessly to his own system and has been known to take a card when he's got 12 against the dealer's two. This is poor strategy, odds wise, but he insists it isn't. There is often no convincing him otherwise. In other words, he can be very stubborn and absolutely certain of his own rightness against the odds.'

As an actor, Marber says, he's the opposite. 'He's very open to direction and I found him a pleasure to work with. He's really shrewd and always serves the writing. He tends to worry but always about things that are necessary to worry about. He's very droll both on and off the stage and he writes funny emails.'

The journalist and writer Victoria Coren worked with Pearson when he played the late John Diamond in a recent TV drama she scripted from Diamond's newspapers columns about his experiences of cancer. She describes Pearson as the antithesis of the celebrated actor, inflated by fame. 'It's so rare that you meet someone like him and he's not, well, a wanker,' Coren says. 'He's properly clever and funny like a normal person.'

It was Pearson's doubts about going on with the film when Diamond died in the middle of the production that convinced her it was safe to continue. 'It meant he was thinking seriously about it; that he wasn't just interested in an hour of Neil Pearson.'

'I've never been interested in an hour of me,' he says, as if I'd just suggested he might harbour lustful feelings for the Queen Mother. In truth what seems to interest him most is the politics of the Left - and thank God because, without it, his press interviews would be horribly stilted affairs that shuttled only between speculation on the private life that he won't discuss and the business of acting that he can't see the point of discussing; it is a grey area, a kind of public-personal business which clearly enthrals.

He is a long time supporter of the Labour Party, though also one of its greatest critics. He admits to being hugely disappointed by the rightwards drift of the current government that he campaigned to get elected. 'I think many people who voted for them would be, particularly when it came after 18 years out of power. What Thatcher was willing to do that Blair seems incapable of doing was be unpopular, years away from an election, when it didn't matter. Blair would far rather do what seems popular than what's right. I find the whole thing politically inept. You turn your friends into your enemies.'

He has a theory about a victory by Iain Duncan Smith in the current Conservative leadership election turning the Liberal Democrats into the party of opposition, and thus forcing Labour back to the left to take them on. Hasn't he ever thought of simply defecting to the Lib Dems? 'I'd much rather drag the party that I've been a member of for 18 years back to where it should be rather than leave it.' To that end he has not been above writing what he refers to as his 'angry of Clapham' letters to the press, attacking stupidity on the part of Government ministers. When the then Education Secretary David Blunkett attacked the play Shopping and Fucking, simply because of its title, Pearson wrote to the press warning anybody who was about to take Blunkett seriously as a cultural arbiter that 'this is the man who, on Desert Island Discs, chose to spend the rest of his life with a Nana Mouskouri record.'

Last year he wrote a longer piece for the Times, attacking the Government's decision to push through Tory cuts in benefit for single mothers, and Harriet Harman's incompetence in announcing it. (When he was five, Pearson's father left the family home in south London and his mother brought her three children up alone.) 'That Harman agreed to be part of this shameful about-turn is, depressingly, only to be expected of a careerist politician,' he wrote, of the benefits cut. 'But the fact that she did it so badly, so unconvincingly; that was the guilt of a mother.'

Indeed, Pearson is an exceptionally good writer. (He has written very funny film reviews from the Edinburgh festival for this paper.) Has he thought of doing more, if he is really so uncertain about acting? 'I can do it but it feels like having to write a school essay,' he says. 'I only want to write when the script I've been given is bad and I only want to direct when I'm being badly directed.' It may be that, for Neil Pearson to finally work out what he wants to do instead of acting, he's going to have to stop getting good parts in quality work and start experiencing some really bad writers and directors.

• 'The Real Thing' is running now at the Bristol Old Vic until September 27, and then on national tour. 'Armadillo' starts on BBC1 on next Sunday , 9pm.