Over the past six years, I've conducted hundreds of hours of subject matter expert interviews, and I made a lot of mistakes along the way, because I thought all I had to do was hop on the phone with a list of questions, ask them, get answers to them, and move on with my day.
This is actually quite a common misconception. Interviewing subject matter experts for content is a bit of an art form, and there are a lot of things you need to think about:
How you prepare for the interview in advance.
What the atmosphere and setting are like for the interview itself; and what you do when you're stranded in a less than ideal interviewing situation.
How to build rapport with an interview subject so they (a) feel comfortable with you, (b) loosen up if they're nervous, and (c) ultimately, give you the expert goodness you need.
Having backup plans and strategies for interview subjects who aren't cooperating or giving you what you need to consider the interview a success.