Give me a roomful of quarrelsome paleoanthropologists arguing over which australopithecines were ancestral to "us", or a roomful of quarrelsome paleoanthropologists participating in the ongoing contentions about who and what Neandertals actually were! It's relatively easy(if you have access to the proper journals) to follow the courses of these paleoanthropological arguments. And access to the relevant papers is relatively easy if you are on some e-mail list where some kindly professor makes some of these available on pdf. I have a bunch of these stored on my hard drive.
Unfortunately, this is often not the case with medieval-themed material, especially the earlier part.
Mirella Patzer an author with two published books to her credit, has written on the difficulty of research into earlier medieval characters. Mirella Patzer writes historical novels, and she seems to be particularly interested in medieval Germany, a place from which a lot of mythic stories seem to have arisen, but which, to me, at least, is a very obscure place. But then, for earlier medieval Europe(approximately from about 500-1000 AD/CE, or a little later), there is very little written material. For this particular time period, the only extant continuous chronicles anywhere are :
Anglo Saxon Chronicle
The Irish Annals
The Russian Primary Chronicle
That's pretty much it. Sometimes there are things like the names of people witnessing charters, and from this you can verify that some half-legendary person actually existed. But it's hard to piece together a historical character from such scraps of information. This is especially true given that the people who recorded history then, were monks, and monks liked to tell "improving" stories, often putting speeches in the mouths of their historical personages. Needles to say, some of these "improving" speeches were never uttered by the people in question. But the conception of history as it existed in those 500 or so years, was quite a bit different from the way we understand historians should write history today.
Also needless to say, this makes it extremely difficult for a modern writer to try to reconstruct a historical personage, for the chronicles may conflict in their assessment(especially if a given monastery was engaged in some sort of conflict with the person they were writing about), or nothing much is known of the person except their name on some charter.
And while I'm not a "strict" historical novelist – I don't exactly consider myself a "historical novelist" at all, I do appreciate the problems Ms. Patzer has in conductiong her own research. If anything, I think her area of research is even more obscure than mine. I'm not boasting here, but I have at least the Anglo Saxon Chronicle, plus one or two other "period" resources to work with; I'm working with early medieval England, not 10th century Germany(which , as I said, I know absolutely nothing about). Still, I sympathize with Ms. Patzer. I've been there, done that, and have more or less torn my hair in the process. This is one of the reasons it took me so long to settle into starting to write the Invaders trilogy. I just didn't have enough information about some of the characters. In a way, I still don't. But I haven't let that stop me. Basically, though it kind of runs against my grain in a way, I've had to invent – invent personalities and motivations.
Also, my main characters are completely fictional, so I more or less have a free hand with them. Some "strict" historical novelists say they prefer to work only with actual historical characters, saying they feel more comfortable with a structure imposed by whatever historical period they're working in. That, of course, is a choice only they can make. But I'm perfectly happy using a mixture of "invented" and real people, even if I have to "dig" quite a bit to find anything out about them. And while this "digging" has caused me to tear my hair during the writing process, it has, at the same time, been very exciting to do it. It has opened up a whole new vista of information for me, and it's ongoing(as is the paleoanthropological research I mentioned at the very beginning). Whenever I discover something new, I try to see ways I can integrate it into my book(s).