There is a comic scene, about halfway through art critic Martin Gayford's patient account of sitting for Lucian Freud for seven punishing months, when the two men are speeding down Bayswater Road in a taxi. Freud suddenly asks Gayford where he can buy bathroom scales, anxious that he may have gained a pound or two in weight. They are about to reach a luxurious restaurant.

Gayford is taken aback, until the artist explains that every ounce counts when you spend 10 hours a day on your feet darting back and forth before the canvas. It must indeed take stamina, Gayford reflects, to be a truly great painter like Freud. "I suggest John Lewis for the scales."

Whatever else this book may be  a series of biographical close-ups, a volume of table talk, a portrait of the artist painting a portrait  it is essentially a double act, two men stuck together until the sittings are over and Freud lays down his palette. And no matter how fascinating its revelations of the artist's working life and his pungent views on painting, what carries the narrative is this dialogue between the two, in which Freud is vividly surprising, potent and dynamic, while Gayford is steadily attentive and respectful, occasionally worrying about his appearance in a mockPooterish manner.

Lucian Freud famously guards his privacy, avoiding the press, visiting museums by night and giving only two TV interviews, to my knowledge, in the last quarter-century. His studio, with its rag heaps, plastered walls and pearl-grey light, appears in most of his pictures, but few people know its address. And though he has talked to other art critics over the years, notably Robert Hughes and William Feaver, Gayford is the first to record the daily ritual of sitting. In his account, a strong portrait of Freud emerges: anarchic, of high vigour and spirits, an accomplished story-teller and voracious painter, working on three or four subjects a day, seven days a week. His conversation glides from Kafka to horses' rumps, from the shortcomings of Christopher Isherwood to Princess Margaret's singing voice, from the genius of Gwen John to the personalities of eggs.

As he works  shielding his eyes, a quiver of brushes between his fingers, dabbing the canvas "like a person making contact with something hot"  Freud mutters and sighs, criticising the latest mark, goading himself onwards. This could be any portrait painter at the easel; indeed the painterly process from charcoal underdrawing to claggy conclusion is the least interesting part of the book, partly because how they are made is so evident in the paintings.

What is unusual is the fact that the sitter is painting a rival portrait, of sorts, and the sheer volume of their conversation. Most of the talk happens before and after evening sessions, "like a marathon dinner date", and Freud's opinions become addictive: his loathing of Leonardo and "the awful Mona Lisa", of Raphael's weightless figures ("I sometimes can't tell which way up they're supposed to be"), of everything by Dante Gabriel Rossetti, whose work is "the nearest painting can get to bad breath". Picasso, whom he knew, is guilty of emotional dishonesty and being out "to amaze, surprise and astonish", compared to Matisse, whose art is far greater because it concerns the life of forms, "which is what art is about, really".

As for the old masters, Freud's insights are piercing and astringent. He cannot love Vermeer for the "curious way his people just aren't there". He believes every good painting contains, indeed requires, "a little bit of poison". It is a pity he doesn't give more examples, but the poison in Titian  his god  is a sense of mortality; precisely what people see in Freud himself.

And about himself, Freud is equally trenchant. The things that many find contentious in his work  his view of the nude as a human animal, of its head as just another limb  are exactly as he ordains them. It is startling to learn that he gave up his early, graphic style as a kind of retaliation to his critics. "People used to write 'He's a fine draughtsman, but the paintings are rather flat.' I thought, I'd better put a stop to that." More surprising still is his ideal response from the viewer of a new work: "Oh, I didn't realise that was by you"; an impossible dream, but remarkably uncomplacent.

Gayford is a careful scribe, noting everything said with plausible integrity (even though, presumably, no tape recorder was used, since he only decided to write the book after the painting was finished). His questions are valuable in everything they yield, and his observations are put over with clarity. He captures particularly well the strangeness of being a model  the terrible consciousness of one's surface self, of the mass of thoughts going on inside the outer carapace, of the disassociation of mind and body. And above all, the peculiar sense that one's looks may be perceived as a surrogate of oneself, just as much as a portrait.

A vain man would never have confessed to everything that Gayford regards as vanity: his disappointment when another portraits gets last-minute attention for a show; his anxiety about whether his ear hair, "protruding in the manner of the late Leonid Brezhnev" will be noticed and recorded for ever. Worrying about a roll of flesh beneath his chin, he keeps trying to tilt it up, hoping Freud won't notice. But he does. It becomes a game of grandmother's footsteps.

The tension ratchets as Gayford yearns for the sittings to come to an end and Freud grows jumpier at the prospect. Few artists have anything very interesting to say on the subject of ending, but Freud does: "The painting's done when I have the sensation I am painting someone else's picture."

After six months, Gayford brightens with hope. "I can imagine this picture finished now." "Oh, can you?" responds Freud. "I can't." The writer goes home, despairing that it will never end because of the unceasing problems caused by the royal blue of his scarf. His wife points out that he has been absent-mindedly wearing two, of different hues.

Gayford's wife also recognises a characteristic darkness of mood in the portrait that he does not. Nor, I suspect, will the reader, by now accustomed to his amiability. The painting is tinged with a smile, but otherwise weighed down with paint, flesh and gravity. Gayford feels it is "me looking at him looking at me", which is only the truth of most eye-to-eye portraits. But what exactly has Freud observed? The eyes are directed outwards but given neither sight nor focus.

A painting or a portrait, a life study or an individual: Man with a Blue Scarf is as slippery a Freud as ever. It is neither a character study nor an exacting likeness (despite Freud's assertion that "What we look like is what we are".) Indeed, the memoir reveals far more about the artist and his model than the portrait, as you might expect. For the painting itself is what matters. "You're here," Freud tells his sitter, "to help it." And so he does, helping it to become a painting of an anonymous man in a blue scarf  another Lucian Freud.

Laura Cumming's A Face to the World: On Self-Portraits has just been published in paperback by HarperPress.