It is important to realize, before purchasing "Improvisation for the Theater," that it will not teach you the silly games and clownish humor you see on "Whose Line Is It Anyway?" Funny though many people find that show, it bears only a shirt-tail relationship to improvisation as Viola Spolin conceived of the concept.
First of all, she probably would have been horrified to discover that many people now regard improvisation and comedy synonymous. In her system, improvisation could have been comedic, tragic, surrealistic, or anything in between. The label hung on the performance was secondary to its quality, consistency, and depth.
In this, Spolin's classic textbook (newly updated and expanded by her son and daughter-in-law, her intellectual executors and heirs), she lays down the ins and outs of improvisation for performance. Activities listed in this book are designed to conduct a full workshop for improvisational actors. There are games listed for absolute beginners, orienting them to the demands of the stage, so there is no false expectation of prior experience. The games, moreover, are almost all adaptable to all ages, so a children's workshop won't feel you're going over their heads, and an adult workshop won't feel they're being condescended to.
The chapters are arranged in the sequence Spolin felt would be most efficient in creating a fully-dimensional improv show that would capture audience attention and be satisfying for all involved. Not everyone will agree that this is the best sequence, and with a little time and consideration, the games can be reordered to suit an individual director's tastes. However, this should be undertaken with care -- many people have used this workshop pattern very effectively for over forty years with great success and enjoyment.
The games, moreover, can be used individually, both in classrooms and in a theatrical directing environment. Many of the games teach important skills regarding vocal technique, character-motivated action, attention to environmental detail, and poise. Even when working with experienced actors, I have found many of these games useful in developing wholly realized characters and environments, and the group nature of the work is key in creating unity among cast members and ensuring everybody is playing off the same rules.
I have worked with scarcely an acting coach or director who has not, at some point, used some activity from this book to achieve some goal. By having actors participate in these activities, the whole production is moved toward a unified and consistent goal, usually one that cannot be achieved by mere talking and finger-pointing. Complex variations of these games are used by improvisational troupes throughout the world, and Spolin's teachings have really been the benchmark for theatrical education and directing for nearly half a century now.
No actor who wants to grow in skill, no acting teacher who wants to guide students toward higher ability, no director who wants to achieve results quickly and well, should ever be without this book. It is the measure of greatness in modern theatre.