There is this aspiring girl writer I am trying to describe. She is just 19 and so broke that she's taken a job that pays almost nothing as a stringer for a British tabloid, the lowest of the low in the journalistic hierarchy. I can see her going down a two-mile hill on a rusty bicycle in southern England. I know she is about to get into one comic predicament after another.

So I know the plot, and it ought to be easy. The problem is that the girl on the bicycle is me.

After nine books and 30 years of writing biography, I am learning about the chasm that separates telling someone else's life from tackling one's own. Putting aside the question of getting a publishing contract and the contrast between selling a story about a famous personality and selling a story about a nonentity (you), the biography and the memoir require radically different approaches, almost a radically different way of thinking. One has to forget everything one has learned about method, tone, construction and denouement and begin all over again.

To begin with, there is the problem of tone. Even if one is destined by nature to be flippant, a biographer will instinctively cultivate a more measured prose if only to demonstrate soberness of purpose. A reader may or may not want the contribution of Emily Blopperstein dismissed, but the writer should at least give the impression of an honest effort to be fair. Indeed, most biographers like their subjects; they otherwise condemn themselves to several years of describing people they secretly detest, which can get depressing. Others, of course, are willing to excuse the indefensible, which is boring and a reason why hagiography is out of fashion, although, politics being what it is, it never quite disappears.

The really successful biographer walks a tightrope between admiring his subject and looking with sympathy on his subject's failings, and he does this with such dispassion that he sets an almost impossible standard for the rest of us. Who could surpass the masterly way Francis Steegmuller described the life of Jean Cocteau, poet, novelist, playwright, artist and filmmaker, along with his subject's contradictions and paradoxes, his charm, generosity and genuinely awful narcissism? Speaking of Cocteau's comment "I am a lie that always tells the truth," Steegmuller wrote, "But a lie, even one that tells the truth, implies a truth that is not told. In Cocteau's case the lie, the myth, and the two kinds of truth, the one that is told and the one that is not, make a fascinating amalgam."

So the writer must be serious-minded but he also has to aim at the truest possible portrait. Despite popular belief, there is no such thing as a definitive biography, which must not stop the biographer from making a determined effort to find out whatever can be discovered. The Quest for Corvo, one of the earliest attempts experimentally to recreate a fascinating and obscure figure, that of the self-styled Baron Corvo, was written by A.J.A. Symons in 1934. Starting only with a manuscript elaborately hand-written by Corvo, Symons worked closer and closer to the inner life of his personality, by turns fascinated and repelled.

He wrote, "[Corvo's] favorite image for himself was the crab, which beneath its hard crust has a very tender core, which approaches its objective by oblique movements, and, when roused, pinches and rends with its enormous claws; but the tarantula spider seems an apter comparison for him as he watched and waited, expectant of the next benefactor." Without falling into the biographer's easy equivocations -- "might have," "could have," "would have" or "should have" -- Symons draws the reader into a dark and poignant drama with only one possible ending.

Biography can use fictional techniques but ultimately has to be constrained by what I call the art of the possible. Memoir, I learn, is the reverse. We admire, even revere, Frank McCourt's memoir Angela's Ashes, but we aren't deceived for a moment. We know this little barefoot boy could not possibly have remembered all the lines of dialogue he recalls so vividly and sneakily suspect that, for the sake of art, he has tidied up and rounded off life's untidy ends and thrown in a laugh or two.

The tone is exactly right: jaunty, full of achingly vivid detail and portraits of eccentrics who are impossible people (but you can't help liking them). There is always a narrator who, for all of his youthful trials and the pain he endures, never ever feels sorry for himself. That is the real trick. I've read it took him 16 years to write that book, and I'm not a bit surprised.

For the memoirist's first requirement is a talent to amuse, as Noel Coward wrote, or if not amuse, entertain. Vladimir Nabokov's Speak, Memory is not consciously funny, but what a storyteller he is, as he comes to terms not only with his aristocratic Russian childhood but also the ravages his own fiction and its distortions have wrought on the self he is trying to rediscover.

His governess, Mademoiselle, is "reading to us on the verandah where the mats and plaited chairs develop a spicy, biscuity smell in the heat. On the white window ledges, on the long window seats covered with faded calico, the sun breaks into geometrical gems after passing through rhomboids and squares of stained glass." This, he writes, is when Mademoiselle is "at her very best," and we are there with him.

That ability to give us the immediacy of a vanished moment is, it would seem, memoir's finest aspect, even if the memoirist must perhaps sacrifice any number of inconvenient facts to propel the story forward. Charmed Lives, Michael Korda's portrait of the Korda brothers -- his father, Vincent, and uncles, Zoltan and Alexander -- takes on the free-floating shape and feel of a novel. Alexander Korda, a pivotal figure in the British film industry before World War II, rubbed elbows with statesmen as well as film stars and could well have received the standard biographical treatment. But writing the story as a memoir gave his nephew free range to ascribe thoughts and motives (after all, he was there), recreate dialogue (who can argue with him?) and, most of all, explore crises and denouements at moments when the biographer, doomed to examine fragmentary clues, would have missed their significance. Above all, Korda has invoked a genre somewhere between memoir and fiction that succeeds in defying the odds.

Then there is an even more daring attempt at the fictionalized memoir to consider, Axel Munthe's The Story of San Michele, a furiously detailed account of the house he built on Capri to appease a ghost. That has to be straight fiction. Or is it?

Compared with writing biography, which is a bit like needlepoint, memoir (as an exercise) has its moments. The whole knotty problem of voice disappears. You write as you think, and if you are perceptive, or funny, so much the better. But then you get to the hard part. If, in biography, the problem is to get close enough to your subject to draw the portrait, the problem with memoir is obviously the reverse. One desperately needs detachment, but how do you attain this impossible feat? Thomas Gainsborough used to tape his brushes to the end of a six-foot pole. Up close, the strokes of paint looked like meaningless dabs, spots and scrawls. But from a distance the results blended into a harmonious and convincing portrait, larger than life and flashing with color. Now, if I can just find a long enough brush. . . . *