Give everyone you interview the same data on you in case they want to add something later or pass you along to another source. Contrary to popular belief, interviews are not social conversations. They are highly-controlled interactions, no matter how chatty or relaxed the reporter appears to be. If you're the reporter doing a phone interview, always ask your subject first if this is a good time to talk and if not, when is. If you're being interviewed and feel uncomfortable, say so.
Every interview is a dynamic between two people, usually total strangers who lack trust.
Explain the purpose of the interview and how much time you need. Most people are more than happy to help once they feel you're dealing with them in good faith.