Writing good historical fiction, it seems to me, requires equal parts of research and imagination. The research is needed for an author to learn as much as possible about the solid, verifiable facts that make a story real: everything from what the hero probably wore (wool? silk? linen? leather? what color, and how dyed? what weight, and how woven? underwear? raincoats? and by the way, where and how did he get it?) and had for dinner (bread? wheat, barley, or rye? meat? pig, cow, or sheep, and roasted or stewed? drink? beer, ale, wine, mead?) to the rulers and boundaries of kingdoms (or other political units) and the probable number, organization, arms, armor, and fighting style of their retainers. Some of my recent reading has included books on food in Roman Britain, horse care and accessaries in the late Roman Empire, the most recent excavations at Sutton Hoo, and the life and activities of St. Columba, to mention only a few.
And the imagination? That's for filling in the gaps, especially when writing about Britain in the Dark Ages (now called the "Early Historic Period"). We don't, in fact, know much about what the hero wore, although reasonable extrapolations can be made from neighboring cultures, where people ended up in peat bogs rather more often. We don't know what color those vanished clothes were, though literature and available dye-plants and processes give some suggestions. We are on slightly firmer ground with the food question, since archeological sites often provide a lot of animal bones and fragments of charred grain, but the lack of pottery of that age from some parts of Britain is puzzling, to say the least. And nobody really knows what sort of language the Picts spoke, although (mutually contradictory) scholarly opinions abound.
The most important thing that a historical novelist needs, or so it seems to me, is a sense of balance and proportion. On the one hand, you should do the research first, so as not to invent things unnecessarily or make obvious mistakes (e.g., tomatoes in your Pictish soup) which will put off better educated readers. On the other hand, you have to remember (or at least I sometimes do!) that this is fiction, not a research project, and that the desired outcome is a novel, not a scholarly article. Although educating the readers along the way (in as unobtrustive a manner as you can manage) is good, at the end of the day the historical novelist's job, like that of any storyteller, is to entertain - and to make the reader want to come back for more.
Did I mention I'm working on a sequel to Storyteller?