The malefactors in Roald Dahl’s fiction are easy to spot. “If a person has ugly thoughts, it begins to show on the face,” the author writes in The Twits. “And when that person has ugly thoughts every day, every week, every year, the face gets uglier and uglier until you can hardly bear to look at it.” Miss Trunchbull, the abusive headmistress of Matilda, is a “gigantic holy terror” with “an obstinate chin, a cruel mouth and small arrogant eyes.” Augustus Gloop, the greedy glutton of Charlie and the Chocolate Factory, is a “big fat boy” who is likened to a dog and a pig. In James and the Giant Peach, the protagonist is oppressed by his “enormously fat” Aunt Sponge, who resembles “soggy overboiled cabbage,” and the “bony” Aunt Spiker, with her “screeching voice and long wet narrow lips.” In Dahl’s children’s books, evil is self-evident and announces itself in crude, stereotypical terms. Giant, a play about Dahl running on Broadway through June, is anything but childish. And although the celebrated writer might be …