Later, hone in on things you need to know. The best interviews are when the subject is doing most of the talking. You're not there to impress them with what you know. It's better to start off nice. The intensity of the interview can accelerate as time goes on.
Biographical questions are good icebreakers.
When the person gets to the important part of the story, slow them down. Ask what they were doing, etc. Resist the temptation to fill in silences. Time will go by, and they'll say it for you. Sometimes I write up all my questions beforehand, especially if the person sets a time limit.