This one of those fundamental, ground-breaking books that re-shapes a field, like Stanislavski's An Actor Prepares. Balk focuses on the world of opera and musical theatre. One of his great insights is to introduce the idea of improv to a form that is notoriously score-bound. His basic program is to integrate singing and acting, and he argues that you need 5 skills to do that: energizing, concentrating, structuring, imagining, stylizing, and coordinating. He includes lots of exercises to build the five skills. A must-have for anyone in that world.