“I can lake any empty space,” says Peter Brook, “and call it a bare stage. A man walks across this empty space, someone else is watching him, and this is all that is needed fur an act of theater to be engaged.” Then there is a flash of magic or there is none, and the results range between genius and failure. The director of MARAT/SADEand other triumphs illuminates the theater scene today in this excerpt from his new book, THE EMPTY SPACE(Atheneum).