One of the biggest pitfalls which can befall the historical novelist is what has been called the "As you know, Bob" syndrome. This is where the writer, needing to give the reader some necessary information, has one character impart it in a conversation with the other character, as in "As you know, Edward, in 1485 Henry Tudor defeated Richard III at Bosworth Field, thereby beginning the Tudor dynasty."
The main problem with this sort of device (which I've been guilty of using myself) is that too often, the character being addressed doesn't need the information: he knows perfectly well what happened in 1485. It's the equivalent of one modern-day American adult telling another in casual conversation, "As you know, Dave, in the 2008 election, Barack Obama became the first African-American president of the United States by defeating John McCain . . . "
"As you know, Bob" often crops up in first-person narratives. I recently gave up on one novel where the author forced one character to give the listening character about fifty years' worth of dynastic information in three or four paragraphs: the result read more like a genealogical table with quotation marks added than a dialogue between two human beings. Third-person narratives, however, are by no means immune to the syndrome. One of the most egregious examples I saw was in a third-person narration set during the Wars of the Roses where two young women, who had known each other for years and who saw each other regularly, were made to recount about ten years of recent history to each other, as if both had recently emerged from comas.
There are some instances when "As you know, Bob" can sound natural. One is when the character being addressed is ignorant of what's being told him, for instance, where a child is being given a history lesson. (This can get dull for the reader awfully fast, though, if not used sparingly.) Sometimes the speaker can be made to have the habit of stating the obvious, thereby putting the dialogue to double duty as both a means of showing character and of informing the reader. At other times, the character on the receiving end can be made to protest, "Well, I knew that already. What do you think I am, a fool?"
A close cousin of "As you know, Bob" is the "your father, the King of England" syndrome, as in, "Mary, you know your father, the King of England, would not approve of these measures." This can work in some contexts, such as where the characters are speaking very formally or where the speaker wants to emphasize the gravity of the situation, but too often it's employed just to let the reader know, or to remind him, that Mary is the king's daughter.
Both the "As you know, Bob" and the "your father, the King of England" syndrome in historical fiction arise, I think, from the admonition to authors to "show, don't tell." The dread of being caught in the act of "telling" means that instead of using narration to give the reader information, the author makes the characters do the job in dialogue. Of course, all the author is doing is making the characters do her dirty work for her.
The key to avoiding these syndromes, and the other literary sin of extended exposition, is to weave the historical background into the story unobtrusively as possible. This is by no means easy, as I can certainly testify; as I said earlier, I've been caught as-you-know-Bobbing myself. I'll keep trying to avoid it. In the meantime, to get it out of our systems, perhaps writers should take a cue from "Talk Like a Pirate Day" and have an "As You Know, Bob" day. What about it?