You must go into the interview with complete confidence. A virtual writing group I know believes in "fake it 'til you make it." Try it. If this is your first or second interview, your subject need not know. The way you present yourself (as an old hat professional, for example) will be the way you're perceived. You can do this.
As for the actual process of speaking with your subject, begin with some small talk. Comment on their cozy office or that great piece of art on their wall. Keep in mind that most people enjoy talking about themselves, so this will likely become a pleasant interaction.
A couple caveats: it is easy to ask leading questions or clarifications. Avoid digging for the answers you want to hear. Another common mistake is interrupting. Quiet down after you ask the question, and keep quiet as long as possible. Determine ahead of time that you're going to let your subject finish complete thoughts, and that you're going to let him or her fill those awkward silences.
