Previous contributors to this series have tended to warn people off writing in the genre that’s being discussed. I could do the same; after all, professionals always like to guard their own patch. But the fact is, an awful lot of biographies get into print, and it’s much easier to get a life of somebody published than it is to make a name as a novelist or playwright or poet.

It used to be even easier. When I started, back in 1974, the biography boom hadn’t really begun. I stumbled into biography by accident. I was looking for a way out of a job in BBC local radio, which I’d done for quite long enough (it involved getting up at 5am to present the breakfast show), but I didn’t indulge in the vanity of thinking I could become a writer. Then one day I was working on a mini-series for my show about J R R Tolkien, who had just died, and whom I had known a little. On the coffee table of the friend who was recording the interviews about him lay a book about C S Lewis - a short biography consisting mainly of photos with extended captions. I thought to myself, there could be a similar book on Tolkien. So I wrote to his publishers, Allen Unwin (one of the old London firms which, alas, has now disappeared).

Rayner Unwin, the chairman, who had known Tolkien for years, wrote back in guarded but not altogether discouraging terms. He said that if I could gain the approval of Tolkien’s family, he’d be delighted to publish such a book; but he didn’t hold out too great a hope of my getting that approval. Nevertheless I set to work, and spoke to Tolkien’s sons and daughter, one by one, until - to my great delight and surprise - they decided that I was acceptable. So a few weeks later, I left the BBC staff, with a publisher’s contract in my possession.

The Tolkien book turned into a full length biography, and, when it was published in mid-1977, it sold enough copies - this was at the peak of the Tolkien cult - to keep me going financially for the next five years. Meanwhile Rayner Unwin was busily making suggestions for other biographies I might write: the Inklings (the group which included Tolkien and C S Lewis), W H Auden, and other ideas. Meanwhile I’d realised, rather belatedly, that biography was something to which I was temperamentally very much suited.

My mother deserves much of the credit for this. She always found people more interesting than ideas, and she used to amuse me with anecdotes of colourful characters she had known or read about. I think this imbued me with an enormous curiosity about what makes people tick. I also have something of the spy about me. As a child, I was always getting into trouble, when we went to stay with another family, for opening drawers and reading any letters or diaries I happened to spot. (I am still unable to stop myself doing this.) Consequently the prospect of spending days, weeks or months in libraries, poring over piles of diaries and hundreds of letters doesn’t appal me, as it does many people.

I suspect that many biographers are failed novelists. I certainly am. Ever since I began to have books published, I have tried to write fiction. Fortunately I managed it for children, but I have yet to achieve a publishable novel for adults. On the other hand I can’t think of any author who writes biographies and novels to an equally high standard. Many novelists have turned their hands to biographies, but the results are usually rather uninspired. A few biographers have written passable novels. But I believe the two genres are really mutually exclusive. The novel requires creation of characters and situations from scratch - a kind of manic free-flowing creativity. We biographers are given our raw material, and though we can certainly shape it in all kinds of different ways, we have to contend with historical fact. Most novelists find that an almost intolerable restriction. Biographers love the challenge of it. At least, I do.

Let’s turn to practical points. Biographical subjects don’t grow on trees any more. They virtually used to. Back in the 1970s, you could still find world-famous characters like Tolkien who hadn’t yet been snapped up by an established biographer. In the 1980s publishers didn’t mind bringing out second, or third, or fourth biographies of the truly famous - so that there was a Hemingway biography industry, for example. That still tends to happen a bit, but the novice biographer won’t be able to persuade most publishers to accept (say) the umpteenth life of D H Lawrence, without having an awful lot of new research material to offer.

c4) = The best bet now, I think, is to look for a fairly obscure character who deserves a biography, but has never had one - maybe a forgotten novelist or poet, an actor, a musician, or possibly an educational pioneer. You won’t find it easy to sell the idea to a publisher (assuming you want a contract before you get going), but you will at least have the advantage of freshness. This really is the hardest part of the job - choosing the subject, and then getting a publisher interested. After writing many biographies, I still find it hard myself. In fact, it’s getting harder, with all the best subjects already in print. At the moment, I’m lucky enough to be writing the authorised biography of Dennis Potter, but goodness knows what I will find after that.

That word “authorised” needs a bit of examination. If you’re writing the life of someone who left lots of written material - letters and diaries and so on - you will want permission to quote from these in your book. Be warned: all the unpublished writings of a deceased person become part of their estate at their death. Letters may belong physically to the person to whom they were sent, but the copyright rests with the author’s estate. So you will usually find yourself approaching a literary executor for permission. This can be tricky. Indeed, I maintain that a biographer isn’t doing her or his job properly if he or she doesn’t get into at least mild conflict with the literary executor. It’s the executor’s responsibility to guard the writer’s reputation; but it’s the biographer’s task to prise open received opinions, and to look under every stone - not necessarily (to mix metaphors and drag in a cliche) searching for the feet of clay, but at least asking cheeky questions.

So: you’ve got your subject, and you’ve got your permission - or you’ve decided to write an “unauthorised” biography, which means you probably won’t be allowed to quote much from your subject’s writings; on the other hand you can say what you think without fear of veto from the executor. Now you have to start research. How?

Lewis Carroll’s advice about beginning at the beginning, and going on to the end is pertinent. I always try to research chronologically. It’s the only approach that allows everything to make historical sense - you see the way that your subject’s personality and ideas grow. Of course, you can’t always stick to the year-by-year approach. If your subject’s mistress is in her nineties and likely to pop off at any moment, better go and see her right away. (This happened to me, in Venice, with Ezra Pound’s mistress, Olga Rudge. She was well into her nineties so I decided there was no time to waste. In fact she lived to 101 and only died very recently.) How to organise your research material? Until the coming of user-friendly computers, index cards were most people’s method. I only stopped using them last year. Now, I’d say the best way is to use a laptop, which you can take to libraries. Simply put all the material you think is relevant - quotations from letters and diaries, bits of published material, transcripts of interviews - in chronological order on a series of computer files. You can easily re-organise them as you go on. And keep a note of the source from the outset. It’s amazing how many experienced biographers fail to do this, and go into a flat spin when they have to provide source-notes.

Before you start writing, read some good biographies. David Cecil is out of fashion these days, but I learnt a lot from The Stricken Deer, his wonderful life of William Cowper. Sylvia Townsend Warner’s biography of T H White is one of the best. I don’t usually enjoy the more modern heavyweight biographers, but read according to your own taste, and try out some passages from your own book in different styles. Above all, treat biography as literature, and not just as a catalogue of facts. Write to entertain as well as to inform. Be true to life, but to art as well. And have fun. I certainly do.

Humphrey Carpenter’s next books are the 50th anniversary history of the BBC Third Programme and Radio 3 (September) and a biography of Robert Runcie (October).

Next week: John Harvey, author of the Charlie Resnick books, on how to write a thriller.