Research obviously is an important part of writing strong, believeable historical fiction. But I'd imagine a lot is left in the notebook rather than transferred to the manuscript. How difficult is it to ensure that the characters come across as real and approachable to a 21st century reader while maintaining their veracity in the historical context?
First off you are right--a lot is left in the notebook and that is as it should be. I think historical novelists need to absorb the period in which they are writing until it is nearly second nature so that when their characters act, speak, dress they will feel authentic. But I think when authors "research dump" the magic is lost. I've experienced that as a reader--gotten to sections of books that feel like the author showing off what he/she has learned for its own sake. I always skim or skip those parts.
As far as making characters approachable goes, I think folks who read historical fiction are pretty knowledgeable and they come to a book understanding that in some ways the past is indeed an alien land. On the other hand what is essential to our humanity hasn't changed, so I believe we recognize the existential struggles of people living in times distant to our own. In fact, I think setting something in the past can make some of life's toughest moments/issues (what does it mean to be a good man? how does one chose between obedience to family and obedience to conscience? what is the essence of love?) more approachable for readers (set in a contemporary novel they might feel too close to home).