Perfection is not the basis of what I'm talking about, says a member of the Cassandra family, which forms the center of Denis Johnson's plays, Hellhound on My Trail and Shoppers Carried by Escalators Into the Flames. The character could be speaking for his creator, because human imperfection is one of Denis Johnson's specialties -- in his critically acclaimed novels, short stories, and nonfiction, and, now, in two brilliant new plays. These...