Books: How to use Harriet's plosive launch pad

You either take actors seriously or you don't. You either think they're a bunch of grown-ups who have yet to earn the name or you accept them as the initiates of an arcane brotherhood. Even within the profession, there are two distinct camps: those who constantly take courses and agonise over every line, and those who walk on to set straight from the bar, without a thought of anything but avoiding major items of furniture.

Harriet Walter belongs firmly in the former - she's a serious woman with a serious something to say about her craft. But why do actors write books? In the old days, they wrote simple anecdotal autobiographies fuelled on name-dropping and backstage high jinks. And then 15 years ago, Simon Callow's Being An Actor triggered the new trend: the actor pontificates, the actor tells everyone else just how it's supposed to be done.

He was swiftly followed by Antony Sher and others, but, crucially, their writing was good enough to sweep you through the process whether you ever intended to step on to the boards or not. Without it, why else should an actor's book of handy hints be any more interesting than the tax inspector's, the dendrologist's?

Now Walter knows her stuff: though hardly a household name, she's done most of the major Shakespeare women at the RSC, Chekhov and Churchill at the Royal Court, Lord Peter Wimsey for the BBC, Sense and Sensibility for Hollywood. In between she's tackled the difficult moderns like Pinter and Berger and Stoppard. She clearly cares passionately about the business and everything it can achieve - her very first job was a South African tour of Marat-Sade which suffered brickbats for breaking the cultural boycott but was in fact cover for the filming of an anti-apartheid documentary.

And when she is describing these incidents, the book is as absorbing as any account of an actor's life can be - fluid and enlightening, chaotic and determined. But then she suddenly remembers that she's not supposed to be writing one of those Then-Binky-Turned-To-Me memoirs, so she reaches for her mortarboard and starts to regiment her thinking. She instructs us in her mental processes: "The most important questions to ask are 'What do I know?' and 'What do I want?'" or "I might use the 'th' in 'throw' as a plosive launch pad." She works us painstakingly through the intricacies of a favourite director's breathing system: "Keep breathing." And she invites inside her dress rehearsal epiphanies: "It is alarming to meet the potential murderess in oneself". You can always tell when she's shifting from spontaneous recollection to serious classroom projection - she drops "you" or "I" and starts saying "one" like a proper grown-up.

Ultimately, she devotes a chapter to each "key" to unlocking a character (that's Psychological, Language, Functional, Biographical ... ). By the end of 300 pages you wonder how you came to be reading it at all - it is no more or less than an actor's handbook, a workmanlike, didactic exposition of technique, of value only to fellow professionals. Dressed up in smart hardback covers, masquerading as autobiography, this book belongs in the Teach Yourself section.

And unsurprisingly, when she takes off her teacher's hat and simply combines memoir with manifesto, that's when the intangible process of acting comes into clearest focus. Preparing with Joint Stock for The Ragged Trousered Philanthropists - a play about exploited house painters in the 1900s - the company found an unused warehouse in Plymouth and spent their mornings redecorating. This gave them a sense of the working life. They spent the afternoons improvising scenes, running exercises, pretending to be anything the director suggested. The result was a series of atmospheres and understandings and lucky inventions which the writer then fashioned into a coherent script. To understand women's poverty still better, Walter knocked on doors in a local council estate until she found a young mother willing to talk to her. (All wasted when she was cast as Bert, the young apprentice.)

Without having seen the production, you never know whether this painting and investigating and game-playing made any difference, but her passionate recollection of those weeks takes you deep into the actor's mind. You begin to understand the boundless confidence they need to establish, their passionate love of pretending, and the desperate search for a route into the morass of unknown emotions which marks the beginning of any production.

But the paradox only deepens as she describes one of her favourite moments in The Price, a Channel 4 series about a woman held for ransom by Irish terrorists. Not having rehearsed the scene because it appeared to be a mere action sequence, she suddenly realised on set that this - the actual point of kidnap - was her "most emotionally naked moment". Playing on her own unawareness and lack of preparation, she caught the perfect response of surprise and terror.

So, Class, what have we learned? Which is better? Weeks of subsidised improvisation and research or just honest-to-goodness spontaneity? Maybe the book's strength and weakness is that she never offers an answer to this question.