Mary Hammond is a voice coach to the stars - Diana Rigg, Daniel Massey, Philip Schofield and Jason Donovan. And now she's working with Brendan Lafferty, Mark Kennedy, Joyce Moar, Harry Fowler and assorted other students at Napier University in Edinburgh.

"Energise! Go further! Go further!" she urges. "Go on, louder! Give yourselves an effort level between one and ten, and treble it! There should be consonants coming out of every orifice you possess!" Hammond, professor of music theatre at the Royal Schools of Music in London, has been persuaded to take a weekend workshop with first and second year students who are putting on a musical, The Beast in the Tower, at the end of this month. Their initial attempts are not encouraging: although they are music students, virtually all of them are instrumentalists, not singers, and some fairly shaky warbling is emerging.

As someone more used to dealing with the Cameron Mackintosh Company, the Really Useful Company and the Royal Shakespeare Company, and coaching the casts of Cats, Miss Saigon and Les Miserables, is Professor Hammond discouraged?

"It's sometimes a bit of a culture shock to be working with actors of enormous experience one day, and young people the next, but look at their enthusiasm. Sometimes you think the task is enormous, but I think it's important to go about and spread what you've learned."

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Musical theatre is completely different from training for classical singing, she stresses, and while it is now tackled in drama courses, it has been ignored in music education. The action in opera is determined by vocal type, such as soprano or baritone, but musical theatre is based on naturalistic acting.

Professor Hammond added: "Your voice has to be able to express extremes of emotion in different ways. You've got to be very adaptable vocally."

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The composer is Stephen Langston, a technician in Napier's music department, and the production originates from a one-hour oratorio he wrote, based on a mythical story about a beast that terrorises Taunton. He outlines a tale of death, horror, treachery and betrayal. "It's actually really funny," he insists.

Hammond is organising the students into doing breathing exercises, vocal exercises, two minutes' meditation. "I bet you're getting tired, aren't you? Now you know why I'm talking about singers getting fit. Do you want a break? I think we should keep going."

She uses characterisation to coax out their singing skills. Mark Kennedy acquires a pompous self-importance as the mayor which gives a comic justification for his not quite hitting the top note.

"He's become more comfortable with the way his character works, so he's singing better and better," Hammond confides. "It's to do with airflow. The psychological effect has a physiological effect which affects sound."

She encourages Brendan Lafferty, the Slayer, to move with more authority. "Can you use a more angry delivery? You've got a good sonorous laugh, but you could make it more pantomimey Ha!'"

An awkwardly static pause, when Joyce Moar as Catherine is seeking the Slayer's help, is swiftly overcome.

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"When people are really attracted to one another, it's difficult to stare into someone's eyes. You actually can't quite face that attraction, so turn away."

She is as concerned with the chorus as the principals. The unnamed villagers all have to decide who they are, and what their relationship is to everyone else, which immediately adds purpose to the crowd scenes.

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"Let's just take a look at the shapes. Could you tell me why you're standing there?"

"I'm with him because he's my friend," explains one young woman. "And she's his wife, she's my friend as well."

"Good job," says Hammond drily.

She drills them relentlessly. "You've got to give some sort of vocal colour to the horror you feel. I know what's wrong! You're not thinking through the rests. There's this dead moment when you're not mentally active."

Langston is astonished by the transformation. "Yesterday, there were 21 people in the room. Today, there's an ensemble."

Hammond is now pioneering a postgraduate course in musical theatre studies in London, aimed specifically at students with a musical qualification.

"When Les Miserables started, it was difficult to find performers able to fulfill the demands of the score. People could act it fine, but were coming a cropper vocally. What I'm trying to get in London is a group of people who are vocally and physically able to express the demands of modern music writing."

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The Beast In the Tower is being performed at Edinburgh's Churchhill Theatre between June 28 and July 1. Tickets from Ticketline: 0131 220 4349.