Historical fiction is set in a specific time in the past, and as such needs to conform to the details of that setting. Characters dress and behave in a manner which matches the time, technology is accurate to that of the period and large-scale events which take place reflect historical fact. For example, a piece of historical fiction set during the U.S. Civil War could feature a character at the Battle of Gettysburg firing a revolver, but he could not use an automatic rifle, nor could he alter the outcome of the battle in any meaningful way. Historical fiction authors need to research the era they intend to write about and double-check their manuscript to ensure that no historical facts are inaccurate.
Characters in historical fiction can be historic figures or they can be fictitious characters created solely for the purposes of telling a good story. In some cases, the fiction may offer a combination of the two, such as a story involving King Henry VIII and a fictitious page boy who served him. In all cases, however, the characters need to behave in a realistic manner which conforms to the conventions of the novel’s setting. If they are historic figures, they need to be accurate to accounts of their life and personality. Their dialogue needs to reflect the speech patterns and phrases of the time--using no modern slang--and adopt the customs and ideological assumptions characterized by people at that that moment in history.
While historical fiction needs to conform to history, it also needs to be a work of fiction; that is, it must convey events and circumstances which never happened. These fictitious elements need to blend in seamlessly with history, without contradicting or invalidating the facts. Ideally, they should illuminate aspects of history which the reader might not be aware of, “filling in gaps” about the way people lived or the struggles they faced and the figures who influenced their lives. But, as with all other forms of fiction, historical fiction should first and foremost tell a good story, with rising action, believable dilemmas and a climax in which the protagonist either overcomes his obstacles or succumbs to them.