As a nod to Dickens, whose genius he has honored in one-man shows on both sides of the Atlantic, actor-playwright Emlyn Williams might have called his first and only novel "Greater Expectations." Like Pip, the hero of "Great Expectations," Welshman Jack Green, the hero of "Headlong," grew up in the countryside and was suddenly handed privileges that tested his ability to remember his humble beginnings. Jack's ancestry is mysterious, but it eventually becomes clear that his genes are a form of privilege. Presumably it is the genes that induce him to revolt against a prospective job in the clay pits, "borrow" a minor sum his mother entrusted him and exchange drudgery for a career in the London theater. Jack has a pleasing voice, limber legs that respond lovingly to song and dance and one of those pretty faces that helps an actor fend off starvation in the best of times and the worst.

It is the worst of times when he comes to Depression London in the early '30s. But Jack is an optimist and satisfied that the Bohemian world he has entered is Eden compared to the village he escaped. New moral and esthetic standards color his vocabulary; he uses the word "lovely" of his relationship with Kathy Tripp, an actress whom his village in a more vehement day would have burned at the stake.

A national disaster introduces Jack to a rarer and perhaps more dangerous world than his Bohemian one. The explosion of a dirigible kills off the king and queen and a whole cluster of their heirs and leads to a manhunt for a legitimate and suitable monarch. Digging among papers that Victoria had suppressed, investigators learn that one of the many secrets of Edward VII's youth included his marriage to a Miss Pasco, that she died young and virtuous, and that Jack is the only living descendant of their union. He becomes King John II and discovers that being a quick study is useless when you have no script.

It's a mark of his wit that suffering, especially his own, inspires him to throw off nice one-liners. "My crown went to my head," Green admits after the splendors of Buckingham Palace have lost their initial fascination and he realizes that for all the kowtowing of lackeys he is not in Ruritania. The awe he felt in the beginning is reflected and maintained by his mother. She makes a point for all anxious mothers when she writes to him: "Your Father & Self baint in any doubt that you will behave as to the manner born & be a credit to us & to your Empire. It is a big relief to us that you are in Buckingham Palace & not in Jail."

A conviction that the palace is in fact a prison grows on the young king. Impossible barriers separate him from his friends in the theater, lackeys at his elbow reduce his privacy to nothing, and insufferable advisers like Sir Godwin -- Sir God to his detractors -- make it clear that their suggestions are to be treated as law. Green had assumed that in the highest echelons of government the officeholder was quite independent of his office and able to define the boundaries of his power. And when he ignores a speech Sir Godwin composed for him, the king puts his heart and mind -- and shows a surprising quantity of both -- into a broadcast that is universally recognized as brilliant, candid and modest.

In the light of British history John II's views on the relations between king and Parliament are regressive. He would nibble away some of the powers of Parliament on the ground that the political parties do nothing but collect prejudices and fight for nothing more idealistic than their own survival. The king, according to Williams' hero, is above politics. Can he remain above politics, however, if he jumps into battle on behalf of causes? And isn't he as capable as his inferiors of turning opportunist and courting the Wrong People to insure his survival?

John II, it must be confessed, is a hero; he would undoubtedly be a credit to Ruritania, and if there's no evidence that he ever handled a pistol or sword, on stage or off, he is gallant enough to risk his survival by defying Sir Godwin and other members of the Cabinet. Unlike Pip, he insists on remembering the past and the miseries from which no merit of his own removed him. In a broadcast he announces that the Treasury holds millions of pounds and that England will enlist vast numbers of the unemployed in make-work projects. It is as easy to be destroyed by our virtues as by our venalities: Such, we may assume, would be Williams' comment if he chose to moralize about the compassion that marked the broadcast and led newspapers across Britain to demand their king's abdication.

"Tell me," an author asked Williams after the production of his thriller, "Night Must Fall," "have ye always had this morbid streak?" There is no psychotic killer in "Headlong," but there are characters who thrive as hirelings. Their maneuvers cause the king's entrapment in a way that Dickens the melodramatist and serial writer might have admired. In the main, "Headlong" is a comic novel with something of the exuberance and taste for satire we associate with the author of "Great Expectations." And like Dickens, Williams writes with fetching warmth whenever he touches the experience of theater people.