Essentially, I'm untrained, so I just go with my imagination and try to put myself as solidly as I can into the shoes of whatever person I'm going to be playing.
Time spent researching varies from book to book. Some novels require months, even years of research, others very little. I try to do most of my research before I begin but inevitably questions emerge during the writing.
We are all human and fall short of where we need to be. We must never stop trying to be the best we can be.