I was on the telly. BBC4, last Thursday, 9pm. Didn't watch it, of course, and it wasn't really me anyway. But I read the book. Dirk Gently's Holistic Detective Agency. Actually I'm in the book. Didn't you know? I'm him. Dirk. Douglas told me. Told everyone else, too, whether they asked or not. Lots of people have been put in books but not quite so blatantly. Who was it who said an author is someone who sells out his friends? But at least they usually deny it absolutely.

Not Douglas.

Examine the character of Dirk Gently. The bits of him that aren't full of bullshit are occupied by hot air. He alternates between arch and the gnomic. He depends on outrageous coincidence propped up by a pantheon of dei ex machina. He's a fraud, a welsher, an exploiter of old women and financially he's as dodgy as a banker. He wears a loud, houndstooth Cheviot tweed and a burgundy fedora. Into his life harpsichords come and harpsichords go. He's a quantum fantasist. He's actually Hungarian. He smokes. He has many more failings, but that is the top-dressing.

You wouldn't model yourself on him. Imagine if he were modelled on you. (Imagine, too, playing him – which I did in The South Bank Show – while knowing he was modelled on you. Spooky. Me and John, we know. Malkovich.) The fact that much of it's true only makes things worse. Harder to shrug off. Impossible to deny entirely. It's like that awful bit where you take your girlfriend home to meet your mother and she gets out the photograph album. Except it goes on forever.

I have no idea when I first become aware of it. Dirk Gently's Holistic Detective Agency – the book – involved, oddly, rather less fuss than Adams's other stuff. It seems now that he just sat at his desk and wrote it, which can't be true because he never sat at his desk and wrote anything; but I can't pin down a moment of sudden realisation that I was being traduced, mocked and insulted with that particular savage but affectionate contempt with which only the English treat their close friends.

But an evening comes to mind, at a now-gone restaurant called Monsieur Frog in Islington (of course). I was going out with a woman at the time who wasn't really going out with me. By looking up Ambition by this newspaper's very own Julie Burchill, I can tell you exactly when it was, too: October 1989, because this woman who I was going out with but she wasn't going out with me was also the alleged model for Ambition's extravagantly calculating sexpot heroine. "Well," she said as we squinted at each other across the third bottle, "there you are, starring in The Long Dark Tea-Time of the Soul. And here I am, starring in Ambition. What I want to know is: where's our money? Other actors get paid. Why don't book actors?"

A fine question, and I suppose the answer is that if they did, there'd be no books. Authors wouldn't cough up. There are probably people who'd pay to be in books, but they'd be so dull you couldn't write about them. So here we are: you parody and savage your friends, manipulate composites of people you've bumped into, and if you're challenged, deny it. Except Adams didn't. "Yes, it's him," he said gleefully. Or indeed, "You know it's based on Bywater, don't you? Read it again. Read the bit where the bailiffs come to repossess the harpsichord." (They didn't. It wasn't my harspichord. I'm not sure where mine is. Somewhere, I suppose.)

It's not just the money. Actors get a say. They can say, "I don't think this is consistent with the character." They can say, "I don't do full nudity." They can say "No." Novel actors don't get the chance. But we do, I suppose, get a sort of twisted immortality: not necessarily the nicest sides of our characters, but this is Parnassus we're talking about, not the local Rotary Club. And while any fool can put himself into his own book, it takes real peculiarities to force your way into someone else's. From which I take a strange sort of comfort.

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