Have a script but be spontaneous too: Write questions in a narrative order to give the interview flow, but be spontaneous too. Let new questions be triggered by what the interviewee says.
Avoid judgment. This is crucial. If a subject feels judged in any way, he or she will almost certainly shut down.
Listen! This is one of the most important, most obvious and most overlooked interviewing skills. It’s easy to think about what you’ll ask next, but some of the most illuminating segments of my interviews come from spontaneous interaction. Stay with your subject and follow him or her into unexplored territory. My interview with Pico Iyer is an example of this. As they say in Vegas: “you must be present to win.”