Many years ago, David Lowenthal* wrote that the past is a foreign country. It has a different language and culture.
If, as authors of historical novels, we are to operate convincingly in that place, we need to learn the language and culture.
The best way that I know of to achieve this is through immersion in the period. I read broad histories of the period, for an understanding of it in overview; a multitude of web-based sources; and autobiographies and biographies, rich with the voices of the time through quotation.
Autobiography and biography let people from the past speak to us in their own words. They reveal cultural morés, emotions and thoughts. They provide detail! The insight into the people, place and Zeitgeist of the period provided by these primary sources is gold. Dig for it. Mine it. The time required to do so is worth the effort. Your stories will have the ring of authenticity that readers will love.
My 2013 novel, The Persuasion of Miss Jane Brody, was inspired by a biography of Mary Wollstonecraft, which led me to ask a number of ‘what if’ questions, such as "How would a woman who believed, like Wollstonecraft, in the equal rights of women and advocated educating girls, cope with falling in love; not with an intellectual like herself, but with a conservative aristocrat?" I just burned to tell that story. Such is the power of biography and autobiography to illuminate and inspire; and readers appear to like this approach too.
* David Lowenthal, The Past is a Foreign Country, Cambridge University Press, Cambridge, 1985. (multiple reprints)