You do as much background research as you can, both the narrow and the broad, into the person, their lifestyle, and the times in which they lived.
If there’s not a lot available about them, then you research the people who interacted with them ? their lifestyles, and the people who in turn interacted with them.
You dig and then you dig some more. This way you build up the layers in the picture and get a feel for what’s right and what’s not.
… If you do the research in enough depth, your story will have the integrity that does history, you, and the reader justice.
How you utilize your research in the novel is down to your personal skills as a writer. Both story and history need to come alive for the reader and shine. No one can be 100% accurate and as writers our imagination is perhaps the most essential tool in our kit, but integrity matters I think.
If you are writing about someone who actually lived, then you keep as close to their personality as you can and portray their world as it actually was ? or as close as you can get, and that includes attitudes as well as furniture. If your characters are imaginary then the same. That’s my take on it anyway – for what it’s worth :- )
I’m in Paris now, doing research. So much rewriting ahead! As always, I find on-the-ground research essential.