Susan Higginbotham has a post on accuracy in historical novels that I find very interesting. There is always a question in historical fiction of how "historical" it should be. Of course there are as many opinions as there are readers. Some people just treat the work as pure fiction using a previous time and real people, and don't expect much. Others at least hope that there is nothing misleading or impossible in the work. And then there are those who expect it to be a fully realized scholarly work that nears the genre of narrative history. I had one reader of an early draft of Saint Mark's Body insist that I should include a disclaimer that clearly stated the parts that were true and the ones I made up.
She doesn't read much historical fiction.
Anyway, it caused me to examine the characters in my book who were real people, and the liberties I took with them.
Buono and Rustico, the Venetian merchants who according to legend stole the body: I've never been able to find a thing out about them other than their names, and that Buono is also called "Tribunus" in some legends. I made them fully developed characters with personalities, which I have no reason to beleive accurate. However, it is likely that I do so without fear of contradiction.
(the future) Pope Gregory IV: biographies paint him as an ally of the Franks and favored member of the Roman ruling classes. He's only in one scene so I don't need him to do much. I chose to have him sort of gently push things in the background, and created a purely fictional character to bring in some of the heavier intrigues.
Emperor Michael II "the Amorian" : A lot has been written about this man - a soldier, probably illiterate, constantly at odds with the workings of the Byzantine Empire he rules. Michael had a lot of trouble both internal and external in his reign. I tried to present him as straightforward, uncomfortable with the byzantine nature of the Byzantine empire, and easily frustrated.
Caliph Al-Ma'mun : Here a lot of writings survive (thank you, literate medieval Islam) about not only the politics and interests of Al-Ma'mun but also his character. I felt that I knew him reasonably well, and was quite comfortable giving Al-Ma'mun words and actions according to his character as I understood it. Interestingly, despite him appearing in only two chapters, a couple of my draft readers said he was their favorite character.
The Doge of Venice, Giustiniano Partecipazio : His history is known, but I have found very little about his character. So I made him what I needed him to be, and tried to think about how the known events of his life (his brother jumped him in the line of succession to the throne, so he deposed his father and brother.... then later on made that same brother his heir!) would have shaped his character. One nice thing about Giustiniano is that his will survives, so we can at least see what he was thinking in the year of his death.