Those accustomed to the genre probably have a more extensive vocab compared to the average person. There are also lots of words that you can infer based on context, or that you at least have a vague notion of to allow you to understand the broad meaning.
However, when reading books written roughly before 1850, I still find myself looking up definitions fairly regularly. It depends largely on the author, genre and style. How often do you do it?
by Teritus12