John Buchan’s The Thirty-Nine Steps is a spy novel with some plot twists. The characters are largely stereotypes that help advance the plot. The hero, Richard Hannay, is a stock character. At the outset, he is rather irresponsible and at loose ends, and he is forced to solve the mystery only to clear his good name. The book was made into a film by Alfred Hitchcock, who went on to develop this falsely accused man as the quintessential hero in many of his films.
In Buchan’s novel, the characters are hampered by the constraints of the plot from developing in any significant way. The reader assumes that the hero is innocent and wonders how he will extricate himself from the dangers in which he becomes entangled: this provides suspense. Along the way, we anticipate that there will be coincidences; as these add up, they provide clues that help the reader solve the mystery. In sum, the characters are only as believable as the author needed them to be.