The way you address someone says a lot about how you think about him or her, and what your relationship is. Just think of the difference between calling someone "Biff" and calling him "Dr. Edmond Von Trapp". This can be a potential minefield, especially when meeting someone new. If someone makes a mistake, it usually takes one of two forms: Referring to a third party without appropriate title can also cause a character to be brought up short. Usually this is also a demand for respect, though it can be a friendly warning that the familiarity might cause trouble.