Patsy Rodenburg is the most respected voice coach in Britain. In her work with the Royal Shakespeare Company and the Royal National Theater (where she heads the voice department), she has trained a galaxy of actors, including Judi Dench, Maggie Smith, Ian McKellen and Ian Holm. As director of voice at the Guildhall School of Music and Drama in London, she also instructs newcomers to the profession.

In ''The Actor Speaks: Voice and the Performer'' (St. Martin's Press, 2000), her most recent book, Ms. Rodenburg tells actors that their voices, even more than their looks, may be their most valuable asset. ''This is where the work of a voice coach comes into play,'' she adds. In Dame Judi's foreword, she calls the book a bible and a security blanket. ''If you cannot have Patsy in person, then her book is the next best thing,'' she writes. (Purchase The Actor Speaks here)

''Patsy in person'' is a dynamo of thoughts, suggestions and exercises into action.

She has worked on contemporary plays as well as classics and been called on by groups, like Theatre de Complicite, that place extreme physical demands on actors. As she says in her book, ''I've had to work with actors speaking lines hanging upside-down from ropes, falling in mud, abseiling, clinging to walls, walking up walls and standing on their heads.''

Ms. Rodenburg's work is never theoretical, always practical and includes detailed procedures on speaking certain sounds in particular contexts. (She stresses the difference among plosives, fricatives and affricates, for example). One of her favorite exercises is to ask actors ''to walk the journey of a speech'' in Shakespeare as they deliver it. As she tells the actors: ''Some sections of the speech will hurtle you forward quickly. You might even have to run.''

Increasingly, she has been working in the United States as well as in Britain, offering classes at the Michael Howard Studios, the National Shakespeare Company and other workshops in Manhattan. Before a recent session at the Howard studios, she talked about her approach to what she calls ''the anatomy of the voice.''

''The voice is housed in the body,'' she said, ''so I start by looking at how physical tensions might constrict the body or the breath.'' When someone gets up to speak, onstage or off, she explained, ''certain things start to tighten.'' She monitors and controls the voice by knowing where that tension will strike, like the shoulders or the jaw. ''Even if your knees lock,'' she said, ''you cannot free your voice.''

Clarity is essential. ''Many people think they're speaking when they're really just mumbling,'' she said. She demonstrated by tucking her chin and talking so softly that her words were indistinct. The idea behind her approach, she said, was to extend the range of the voice but not to make that range obvious, in the manner of ''those old British actors who would do that to show off.'' Imitating them, she raised and lowered her voice with forced inflection; it was a seesaw of loud and soft.

Ms. Rodenburg acknowledged that British actors generally speak more distinctly than their American counterparts because they have had intensive voice training. ''It's a tradition that's been respected for 150 years,'' she said. In acting schools in the United States, emotion is sometimes elevated over speech.

Actors who focus their careers on film or television often find it difficult to return to the stage, Ms. Rodenburg said. Removed from an audience, their voices can be neglected.

''It's rather like ballet dancers at the barre,'' she explained. ''Dancers work their body so that when they jump, they're not thinking, 'Oh, I better flex this muscle.' Actors think voice training is going to be complex, but just by doing a bit of work every day, they get to the point where it just starts the motor for them.''

Last year Ms. Rodenburg worked with Ralph Fiennes for the first time in years. ''I had to put him back onstage,'' she said. Mr. Fiennes was preparing to perform in ''Richard II'' and ''Coriolanus'' in London and later at the Brooklyn Academy of Music.

''He was worried that after four years of film, not only had his voice gone a bit rusty and needed working, but he wondered if he could remember all the text,'' she said.

Her work with Mr. Fiennes was relatively easy because of ''the great foundation'' of his classical acting during his early years with the Royal Shakespeare Company, she added. ''A lot of young actors now go straight into film or television and never get to do that work,'' Ms. Rodenburg said. ''It's not embedded in them. When you work with actors who have been onstage for 40 years, the work's been done by their doing it every night.''

Ian Holm presented a special problem. After years as a leading actor on the London stage, he had developed severe stage fright and for 17 years ''just couldn't get onstage,'' she said. Then he starred in Harold Pinter's brief play ''Moonlight'' at the Almeida Theater in London and apparently felt confident enough to take on a more demanding challenge: ''King Lear'' at the Royal National Theater.

Almost daily for a period of weeks, Sir Ian and Ms. Rodenburg worked together ''on the manifestations of his fear,'' she said.

''His tension took him right into his chest and away from his breath,'' she continued, ''and that's when he started to panic. The voice comes from the lower abdominal muscle. I taught him how to breathe again, and how to keep centered. Then he found he could do it on his own.''

Ms. Rodenburg said every actor was scared. According to a medical study, she added, ''an actor going onstage for a press night undergoes the same tension as a victim in a major car accident.'' But her theory is that fear can be used to the actor's advantage. ''That's why they get depressed when they're not acting,'' she said, ''because they're off this huge adrenalin.''

Dame Judi had a different difficulty. Several years ago she agreed to play Desiree in Stephen Sondheim's ''Little Night Music'' at the National Theater. Although she had sung before onstage in ''Cabaret,'' she felt that she was not a natural singer. What she had in her favor, Ms. Rodenburg said, was an instinctive understanding of lyrics.

''She connects to the words, which breaks your heart when she's singing,'' Ms. Rodenburg said. ''In a way, you shouldn't hear somebody's voice, you should hear what they're saying or singing. And that's what Judi does brilliantly.''

In ''A Little Night Music'' ''Sondheim insisted that 'Send In the Clowns' should be sung while sitting on an unmade bed,'' Ms. Rodenburg explained. ''But Judi is very short. She sat on the bed, and her feet didn't touch the ground. Until you feel your feet on the floor, it is very hard to get breath and support. You need that connection. So she couldn't sing.''

The solution was to build up the headboard of the bed and to have Dame Judi push against it. By doing so, she managed to get her grounding and breathe well.

Ms. Rodenburg's work often requires her to deal with special situations: controlling the voice while crying, drinking, coughing or dying onstage. When Nigel Hawthorne did ''The Madness of George III'' at the National, for instance, he had to enter screaming because of an attempted assassination.

''We had to figure out what to do,'' she said. ''Eventually, we timed it when he took his breath and screamed,'' and held to that pattern.

''He reported to me that within about 10 days it became organic,'' she said. ''If you do some of these things wrongly, like screaming, you can do permanent damage to your voice, and that can be the end of your career. So I err on the side of safety.''

When an actor loses his voice, he is sent to a laryngologist, she said. The most dangerous circumstance is when a performer with a damaged voice does not feel pain and wants to go onstage anyway. Preferring not to take a chance, Ms. Rodenburg asks for the understudy to play the role.

Her choice of career is partly a reaction to her own childhood difficulties. At 8, she was sent to an elocution teacher who scared her so much that she was not able to improve her speech. Much later, after studying at the Central School of Speech and Drama in London, she worked briefly as an actress and then taught English. While teaching, she began studying voice and was soon working as a coach.

Although Ms. Rodenburg's lessons and books are directed to actors, they can be valuable to anyone speaking publicly. She is often asked to work with lawyers, professors and other professionals, but prefers teaching actors.

''I like actors,'' she said. ''The good ones are not vain; they're full of humility. When they go onstage, you know immediately if they can do their job. You can be a lawyer or an accountant for years and not find out.''

It was time for her class to begin. Facing a score of actors, she started with simple physical and vocal exercises, asking them to lean against one another and to chant lines in unison. As a hum rose in the studio, Ms. Rodenburg said: ''Just get those muscles going. It's nothing more complex than that. You center yourself. It's giving yourself a ritual so that you can prepare for the next three hours of your life.''

Leading the class, she emphasized trust: trust the words of the play, trust the voice, trust yourself.

Getting Centered

For the voice coach Patsy Rodenburg, finding one's center is an acting principle that can be applied to the body and the voice. She sees it as a practical, not mystical, process. Here is a basic centering exercise from her book ''The Actor Speaks: Voice and the Performer'' (St. Martin's Press).

Stand feet parallel under hips.

Energy slightly on the balls of the feet.

Knees unlocked but not too bent.

Spine up but not rigid.

Shoulders released.

Drop head onto chest; feel the weight.

Let the weight of your head take the upper body over, so that you flop over from the waist.

Shake your shoulders out while bent over.

Check that the knees remain unlocked and the back of the neck released.

Come up slowly from the base of the spine.

Let the shoulders fall easily into place. Avoid the temptation to place them.

The head is the last thing that should come up.

Keep breathing and the jaw should remain free.