The need for verisimilitude is one of the main features of historical fiction that distinguishes it from historical non-fiction, particularly of the academic, non-narrative type. The reader needs to feel he or she is living the history alongside the characters.
Consequently, primary sources written by people living through the events you're writing about, such as diaries, journals, letters, etc., are crucial to complement your background research in secondary sources like books, websites, etc. Not only do they give you a first-person account of your subject matter, they also reveal the mindset and "voice" of people from the time period. Many such primary sources may be available exclusively in local historical societies and archives.
It's a bit of a thrill, too, to do this type of research and hold in your hand the actual pieces of paper used by your character or someone on whom your character is based. That alone can really bring your subject matter home.
Another great way to pursue verisimilitude is living history or reenactment events. Re-enactors typically have researched their time period and "characters" exhaustively. Beyond the familiar battle reenactments, many living history organizations and museums will hold workshops, demonstrations or even encampment weekends that anyone can attend and participate directly in historical activities like cooking, farming, sewing, scouting, etc.
But don't overdo it.
