'Critics," said the Irish writer Brendan Behan, "are like eunuchs in a harem. They know how it's done, they've seen it done every day, but they're unable to do it themselves." Perhaps it was Behan's voice that was whispering in my ear when I recently turned down an invitation to write a short play. It's not the first that's come my way, and of course I was flattered – who wouldn't be? – but I tend to think that, while 30 years of sitting in the dark watching other people's plays has taught me a great deal, one of the things it hasn't taught me is how to write a good play.

Writing reviews and recognising a great (or even promising) play are entirely different things, and I think it's best to leave the latter to the professionals. Being any kind of writer takes practice and, while some of the skills are transferable (I write a mean note for the milkman), being even a half-decent playwright takes craft and graft. Great playwrights do not spring fully formed from their mother's wombs. I recently heard Mike Bartlett, writer of Earthquakes in London, say he wrote almost a dozen plays before his first was produced. You can learn a great deal by watching, which is why theatres such as the Royal Court and the Bush in London provide free and cheap tickets for playwrights. Seeing other people's plays can spur your own creativity, but you ultimately learn by doing. Again and again. Even if I did manage to finish a play, it is unlikely it would be stageable.

That's not to say there haven't been successful gamekeepers turned poachers. George Bernard Shaw, critic on the 19th-century publication Saturday Review, is the obvious example, a man who, after sitting for several years in his seat in the stalls, felt compelled to show people how it should be done. There have been others, too, most recently Nicholas de Jongh, whose play about the homophobic 1950s, Plague Over England (pictured above), ran in the West End; meanwhile, Dominic Cavendish produced a fine adaptation of George Orwell's Coming Up for Air.

I admire their bravery. But it's not just fear of being made a laughing stock that makes me hesitate. While the literary world sees nothing strange about the novelist also operating as a critic, and vice versa, one of the strengths of theatre criticism in this country is its independence. People who might commission a play, or be commissioned to write a play, do not review each other. Of course, I'm in favour of a more developed dialogue between critic and practitioner; but, as someone who is lucky enough to earn a living as a critic, it's less compromising and complicated to keep my creative life and my critical lives separate.

This doesn't mean theatre critics are doomed to simply feed off other people's creativity like vampires. I write novels for children that draw on my night job, and Observer critic Susannah Clapp recently published an exquisite memoir, A Card from Angela Carter, inspired by her friendship with the novelist. Maybe even we eunuchs can find, and give, satisfaction when we find our way to other harems.

Could you all please sit down?

I'm struck by how frequently people write to berate me for failing to give a show a wholly positive review, saying I'm wrong because "the audience loved it". I might indeed be wrong, but criticism is not an exam; it is a subjective opinion delivered by someone, in this case me, who happens to go to the theatre five nights a week.

The popular argument that it must be good because the audience loved it is one I just can't buy. I review what I see on stage, not what the director has told me the show is about in the programme; and I certainly don't review the audience. In recent years, the trend for audiences to rise to their feet and applaud every show wildly would mean that if I did review the audience, almost everything would get a five-star rave.