How do you figure out what type of personalities historical figures would have?
Great question, Jasher- thanks! As with so much in historical fiction, research can help a lot. Even as far back as the 12th century, you had chroniclers at court who wrote about people they witnessed. By reading their accounts, I found out that Henry II, for instance, had a terrible temper and spurned luxury. Hugh de Lacy, Henry's first Lord of Meath in Ireland is accused by Gerald of Wales as being "greedy for gold and more ambitious for his own advancement and pre-eminence than was proper."
Now, while all this factual background information helps, it's then the job of the novelist to make these people come to life. We interpret as many of the facts we have to create our fictional characters. So I create their own life stories up to the point of when the novel starts. That helps me to shape their motivation and their own internal challenges, as well as those that life is throwing at them. And they will often take on a life of their own- even I get surprised at what they get up to!