When you get a great idea for a story, but you don’t know much about the setting of the story, like the medieval times, how do you go about writing it? How do you gain enough information about a certain setting or place so that it sounds genuine (like Fargo) and not fake or phony (the basketball scenes from Finding Forrester)?
You do research. Right now, I’m in the process of writing a show for ABC about murder in Alaska. Before I started working on it, I knew very little about State Troopers, the provincial government structure, the Russian Orthodox Church, tribal corporations and bush planes. I’m certainly not an expert on any of these subjects now, but I know enough to write the pilot.
In my case, I started by reading books and Googling a lot of different Alaska websites. The second round of research involved a lot of time on the phone, calling the various people who actually have the jobs I’m writing about. Finally, I’ll be making a research trip up to Anchorage to interview these people in person, and investigate a lot of intangible details. (Such as, do Alaskans carry over any of the strange Canadian pronunciations, like "SO-ree" rather than "SAR-ree"? When Alaskans refer to non-Alaskans, is there a term they usually use?)
If you were researching Medieval times, you would obviously find a lot of your information in books about the period. But it would also behoove you to find some experts in the field, and even visit some authentic sites to get the most possible verisimilitude.
Just remember that no amount of research can substitute for good writing. Knowing the exact shade of ochre in the king’s bedroom is pointless unless you have a fascinating scene taking place there.
And keep in mind that audiences carry with them certain misconceptions about places and times that make certain details less than crucial. For instance, most audiences think of horses having saddles, and saddles having stirrups. So when we see Russell Crowe and his men charging in on horseback at the start of GLADIATOR, sandals in stirrups, we think nothing of it. Yet as many historians (and Internet nit-pickers have pointed out), stirrups were invented centuries later. Maximus’s advice to "keep his heels down when riding" is impossible.
Also, in A KNIGHT’S TALE, the people wouldn’t have been singing along with Queen’s "We Will Rock You."