The overarching theme of The Murder of Roger Ackroyd and The Moving Finger is the same as that for all crime fiction: the clash between good and evil. The working out of this theme on the side of good constitutes the essentially comic nature of such fiction. Within this theme Christie develops two others in her books. In the Hercule Poirot stories, readers are constantly reminded of the importance of thinking and of order. Poirot eschews the Holmesian technique of getting down on his knees with a magnifying glass to find clues; instead, he uses his "little grey cells" to solve the mystery, often making gentle fun of the other characters for their desire for action rather than thought. Poirot also seeks logic and order above all, even to the habit of straightening pictures, pencils, or other objects, as well as the clues in a case. This passion for order invariably leads him to the solution that others have missed, such as the links among the death of Mrs. Ferrars, the murder of Ackroyd, and the disappearance of Ralph Paton.
In the Jane Marple stories, another theme is developed: the importance and protection of the innocent. In The Moving Finger, a number of innocent people receive poison pen letters; Christie treats their distress with sympathy, and when Miss Marple is brought on the scene to solve the mystery, she must put young Megan Hunter in danger before she can get the murderer to reveal himself. "We are not put into this world to avoid danger when an innocent fellow-creature's life is at stake," Miss Marple says; in several of her stories she puts herself in danger more than once, and in A Pocket Full of Rye she rushes to the aid of a poor servant girl. In the Marple stories, and to a less noticeable degree in all Christie's crime fiction, the innocent must be protected from the suspicion of guilt, for such suspicion finally destroys happiness.