Authority in Documentaries

1. What are the visual conventions of experts' appearance, behavior and setting? i.e. What do you think they all have in common? They all look mature, grown up, obviously you wouldn't have a 20 year old 'expert' otherwise it won't make the viewer believe in you. The 'expert interviews' are usually set in libraries or the setting of the fact because it makes the viewer see more authority, for example, if it is set in a library, viewers might think that because there are a lot of books near the person speaking, they're reliable, because books are a symbol of knowledge to many people. 'Experts' are all very calm because if you are hesitating you wouldn't look like you are telling the truth, they all speak very slowly and paced. 
2. What are the positive and negative consequences (for both producers and viewers) in the use of these conventions? It would make the producers loose a lot of money when the viewers find out that what was said was fake and unreliable, and the viewers would be forced into believing in the wrong thing.
3. What strereotypes of people are most commonly used as experts? Who is rarely shown?  Why do you think this is? People believe that teachers, professor looking characters are suggested as experts because people believe that they are the people that have studied the topic and have to be correct. People that are never used as experts include characters that don't necessarily look like experts to most people like : kids, teenagers, field workers etc.