Characterization in hard-boiled detective fiction is typically not overly developed, as the emphasis is on investigation into events rather than character analysis.

The genre requires a detective (in this case Millhone), and a criminal (Bethel), and other characters are encountered mostly as conduits for information. O Is for Outlaw possesses a rare character, however, in the form of Magruder. The discovery of his behavior and values is at least as important, if not more, than the discovery of the criminal.

The character of the detective is part of the genre's constitution, and its foundations go back to the nineteenth century and Edgar Allan Poe's tale, "The Murders in the Rue Morgue" (1852) in which Dupin makes his appearance. Along with Arthur Conan Doyle's Sherlock Holmes, first appearing in 1888, Dupin demonstrates the utility of rationalism in the face of the inexplicable. The first female detective created by a woman writer...