Provide a focus for discussing why you're doing this interview and what you'll do with it. Address any concerns they might have about confidentiality, access, and so on. Clarify any restrictions they might want to place on the interview. Let you know when they're ready to have you turn on the tape recorder. Most people love having a chance to talk about themselves, and very few people will reveal things they don't really want to reveal.
Many people get tired of talking after two or three hours.
How much talking you need to do in order to keep things going will vary. Use whatever works. There's no taboo on a certain amount of dialogue on tape, but remember that the interview is about them, not you. There's nothing wrong with silence. If you don't understand something, ask.