Don't launch into a lengthy description of how important it is to you to find records of great-grandpa, or start telling about your family history. An expression of gratitude takes no more time than a disagreeable expression and a rude manner. Observe closing and lunch hours. There is no point in demanding death records, if these are kept only by the state. Don't insist on finding marriage records before the time the law mandated they be recorded.
Good manners are important in genealogy just as in other aspects of life.
You may be smoothing the way for the next genealogist to visit a clerk's office. Well said, and excellent advice. And, remember, you may have to make a return trip someday. Before you make your next field trip, read it.