The Caine Mutiny is primarily about the changing character of Willie Keith. At the beginning of the novel, Willie Keith is immature, weak, and spoiled. Most of his actions are dictated by a desire to disconnect from his protective mother. Instead of putting his Princeton education to use, Willie becomes a lounge piano player. Willie is also unwilling to commit to his wonderful girlfriend because she does is not the rich, well-bred girl Willie's mother imagines for her son.
Willie slowly matures over the course of his Navy career. De Vriess immediately recognizes the potential beneath Willie's irresponsible, unmilitary exterior. By the end of the novel, Willie has become a man by dint of surviving the hardships of Queeg's command of the ship, the trials of war, and the risk of calamity and death. Just as Willie slims down because of the Navy food, he sheds his immaturity because of the Navy experience, and becomes a resilient, brave, and reliable man. Some would even call him a hero. Willie makes three major steps in his development. The first one comes in response to a letter in which his father observes that Willie has never had a major test in his life before the Navy, and how he fared would have a major bearing on the rest of his life. As a result of this letter, Willie determines to succeed in the Navy. The second step happens during the court martial, when Willie realizes that he acted to remove Caine from command out of personal dislike for the captain, not out of worry for the ship. He realizes that this was a mistake, and from then he is far more objective. The third and most important step on Willie's path to manhood occurs when the kamikaze strikes the Caine and Willie understands the one thing that he regrets and decides to rectify his fault.
At the center of all interpretations of The Caine Mutiny lies a debate over Captain Queeg's sanity. It is particularly difficult to assess Queeg's sanity because we see his actions through the eyes of an unreliable man. During the typhoon, it seems clear that Queeg is insane, but later, when Willie reveals in the court martial that his impressions were tainted, we have to reexamine the event from a more objective point of view. We must also wonder whether Willie's subjectivity colored the rest of Queeg's travesties, many of which seemed insane.
While many of Queeg's acts are rash and cruel, none are absolutely indicative of an insane person. Queeg is unsuited for the pressures of command, simpleminded, stubborn, paranoid, and confrontational, but he is not necessarily insane. He performs admirably in the routine elements of his command and would have made an excellent commander in a peacetime Navy. However, Queeg performs badly under the extreme circumstances of war when asked to perform tasks for which he is unprepared and to command sailors more educated, clever, and skilled than he.
Although she is not present in the novel very frequently, May Wynn affects almost every event of the story. Like Willie, May does not think much of their relationship in the beginning of the novel, but she cannot let it go when Willie leaves for sea. She is less reserved than Willie is, directly confronting him with the difficult truth about her feelings. When she kicks Willie out of the car in front of Furnald Hall, she suggests that she wants either to marry Willie, or to cut off all contact with him. Because she prefers the option of marrying him, May gives Willie a chance to recognize how special their relationship is.
In Yosemite, May's conscience plagues her. The novel sometimes delves into her consciousness, revealing that May might have planned exactly what Mrs. Keith later suspects: to sleep with Willie in the hope that it will trap him into marrying her. When Willie proposes, however, May's conscience does not allow her to accept him. At the end of the novel, there is a suggestion that May Wynn will accept Willie's marriage proposal, which has become the wholehearted, guilt-free pledge of love that she always wanted from Willie.