Writing believable historical fiction requires at least some idea of the customs, politics and way of life of the period you choose to write about. The importance of such knowledge varies, depending on genre and storyline, but getting the basics right is vital for your credibility as a writer.
In general historical fiction, where the politics of the day might interfere with the MC's life, it is crucial to get your facts right. And there are no excuses! Nowadays, it's fairly easy to read up about kings and queens, political parties and the differences in roles between nobility, gentry and the working classes. There are huge resources available online, that even just a decade ago seemed unobtainable. And, of course, there's always the fat volume of a history book to browse in and take notes. Most libraries can get you copies of the most relevant tomes.
Without turning your story into such a history book, you would want your characters to be as close to the real thing as you can make them, so ensure to read up on their types of behaviour, clothing, setting and day-to-day customs. Explore how they would address each other, and their superiors or villeins. Ensure that, if you use real life characters such as kings, they were indeed known to be in the area you set them, or at least that there is no record of them being elsewhere. It only takes one reader to know the facts. Join interest groups of the same genre to swap information. Many of those groups are fountains of knowledge and the members are always happy to share details and tips.
In historical romance, research is still of importance, even though the plot would focus on the two MCs rather, with the external influences of the day filling out the background. Yet it can jar a reader's enjoyment when details such as address, clothing, assumed behaviour or a monarch's name are out of synch.
Never underestimate your readers! They come from all walks of life, with many readers of historicals having gained huge insight into what was acceptable during a certain age - and what seems totally out of place. Some don't mind if you get things wrong, but others might throw your book against the wall. And we don't ever want anyone to do just that...
Shortly, I'm going to start posting links to helpful websites and groups for historical fiction, romance and mystery research. You may also find other, more specific groups of enthusiasts in yahoo or google groups. Go and ask to join them! You won't regret it.