We discussed whether one could get into a character's head if they lived at a different time. Rosemary Aitkin, the other panellist, who (as Rosemary Rowe) writes crime novels set in Romano-Britain, said that it was impossible to write about accurately, the writer had to adapt it. Real life Romano-Britons would have had a mindset about, for example, the place of women in society, that would be unacceptable to us now.
Closer to home, I find it hard to understand the mindset of many young people, for example, about drinking. I just don't get the idea of going out with the intention of getting wasted. Looking the other direction, I know my mother's expectations of marriage were quite different from mine, and what I might consider to be a bad marriage, she might see as a successful one.
If we want to write about characters other than ourselves we have to examine our assumptions about other peoples behaviour. We don't have to understand them - I am never going to understand why cars matter so much to some men - but we have to know that they're there.