One similarity could be the basic premise of the story. If in the true story someone asked for a'simple favor' that led to a complex web of events, the novel would likely retain that central idea. Also, the general setting or the social context in which the events take place could be the same. For example, if the true story was set in a small town and involved a certain type of community, the novel might mirror that. And the relationships between the characters, like friendship or enmity, may also be modeled after the real - life relationships from the true story.
The main characters' personalities might be similar.