So you have this great idea to set your next erotic masterpiece in the Restoration Court of King Charles II, complete with Nell Gwyn and a bare breasted Lady Arabella Churchill. Fine I reply, "what did Nell Gwyn do for a living?"
"She sold oranges outside the theatre."
I am impressed that you knew the theatre part. At this point you are doing quite well. Then I say. "Which theatre?" Your face is perplexed. I am no longer impressed, if you want to write historical fiction you need all the historical facts at your finger tips.
This article relates to Historic fiction, not to myth or fantasy fiction. By Historic Fiction I mean a story that is set in a defined time and place.
Myth or Fantasy Fiction stories are set in some very vaguely defined timeframe in undefined or imaginary locations. Thus the writer is free to invent not only the characters but also the culture within which they exist, an example of this type of fiction would be Arthurian tales from the era of chivalry. Thus these tales require little or no knowledge on the part of the author. If you want to write this type of erotic fiction read some of Aran Ashe or Anne Rice's work. And it looks like lots of fun to write, like Sci-Fi you will be able to create a form of parallel world.
The writer of Historical Fiction has no such luxury of freedom. All writers regardless of genre have to strive for truth, to convince their readers that no matter how fantastical the tale the story being told is true. If the writer of Historicals deviates from the known truth, by telling unsubstantiated untruths the reader will not believe in the authors honesty. So the story being told will be constrained by historical facts.
Historical fiction comes in many guises; the loosest form creates a complete cast of fictional characters. The tightest form inserts a fictional character and or events into a true story. In the second-case the writer is probably trying to advance some pet theory to account for some inexplicable action on the part of the main protagonist. e.g. Why in 1940 did Hitler halt his victorious army and merely encircle the British at Dunkirque instead of annihilating them? (An often asked question) Now say the theory you want to advance is that he was too busy indulging at an orgy! The military history must be correct, as must Hitler's known timetable and movements. The idea will not work unless there is a documented unaccounted gap which creates the opportunity to create the fiction.
By now you are protesting I only want to tell a dirty story set in the California Goldrush of 1849; A simple Jack/Jill Off story.