Here are some of my rules for writing historical fiction. They may not apply to all writers, but they guide me in the choices I make and the kinds of research I do. Read them first. Then later this weekend, I'll show you how they helped me choose which of my diary copies would become my writing guide.
1. Be true to the time period.
Don't ever guess at the order in which events took place. Double-check dates and times so that you don't run a chance of turning a cause into an effect. There's a difference between saying that a man shot a dog because the dog attacked him, or that the dog attacked the man who tried to shoot him. In the first instance, we're dealing with a vicious dog; in the second, the man may be the one who is vicious.
If your story is about people who live in a particular time period, be sure you know the appropriate details of dress, food availability, household furnishings, modes of transportation, and social customs of the period. Also check details of local vegetation, climate, and wildlife habitats. Don't let your native of Oklahoma pull a salmon out of the local river.
If your story also involves actual political or military events, your responsibilities multiply. Your descriptions and discussions must reflect the facts as they were known at the time. Don't let hindsight lead you astray here. We now know that a pregnant woman who takes the drug thalidomide runs a grave risk of birth defects in her unborn child, but the doctors who prescribed the drug to cure morning sickness back in the 50s did not. Don't blame someone for lack of knowledge if that knowledge was unavailable at the time.
2. Be true to your story.
Most historians hate playing "what if" with history. No matter how many alternative universes you may describe, it won't change the one in which your events actually took place. What if Germany had won World War II? Maybe Hitler would have managed to turn the entire world population into blond, blue-eyed Aryans. Or maybe he would have turned out to be a really nice guy whose genetic experiments resulted in the cure of cancer and other life-threatening diseases. Or maybe he would have been hit by a bus and we would have discovered that we didn't need to fight that war after all. Now we're talking fantasy, not history. And while fantasy may be amusing, it doesn't increase anyone's understanding of anything.
Don't change the facts to suit your story. Change your story to make it fit the facts. The people who read historical fiction may be people who know the period well. Or, if they don't know much about the history, they are probably hoping to learn something from your story. It's foolish to try to hoodwink the first type of reader because they will just dismiss you as clueless. It's unkind to mislead the second type of reader, because you will be betraying their trust. Either way, you will lose readers, not gain them.
3. Most important, be true to your character.
If you are writing about a real person, you owe it to yourself and to her to find out as much as possible about her. Don't exaggerate her education or experiences. Work with her own life to make her struggles more understandable. Don't rely solely on gossip or what others thought about the character. Ask what she thought about herself. That's why diaries and personal letters are so helpful when you are trying to flesh out a character.
Judge the characters in your story only as you could have judged them in person. You must not criticize someone who made a well-considered decision simply because it turned out badly. You need to look beneath the result to discover the intention. Don't just blame Lincoln for not acting earlier to emancipate the slaves. You must try to understand what he hoped the Civil War would accomplish before you can judge his efforts. Before you judge a slave-owner, you must at least try to understand why he needed to have slaves in the first place. Only then can you start to examine his treatment of those slaves.
Finally, let your characters be real. Nobody's good and kind all the time. We all have thoughts and temptations we're not proud of. We all have weaknesses. At the other end of the scale, nobody's pure evil. The meanest boss may have a penchant for big-eyed puppies. A kid who terrorizes the neighborhood has a mother who loves him. The heartless mother will willingly sacrifice her life for her child. Don't try to gloss over the unattractive elements of your character's personality. If she's perfect, everyone will hate her by the end of the first paragraph.Your readers want real people -- people with whom they can identify, people they understand because they recognize them.