Why do "everyone suck at their first research interviews"?
Well, this is not exactly an answer to your question, is something much more like a long comment.

My suggestion would be to try with some mock interviews before going to the real ones. The purpose is to make you feel comfortable with the interview process.

The first step is to create some checklists. What you should ask? What you shouldn't? What you should ask to the interviewee if he/she answers something specific to a specific question? What you should ask if he/she asks something different instead?

The first step would be to interview yourself in the front of a mirror (seriously!) What would you answer if somebody else interviewed you instead?

The second step would be to interview the devil's advocate. This role could be accomplished by yourself impersonating the devil's advocate or a trusted friend doing the job. The devil's advocate would be someone who would give answers that are:


strange
unexpected
trollish
useless
lying
confusing
unintelligible
contradicting
lunatic
irrelevant
incomplete
bizarre, surreal, alien-like
agressive, threating or unrespectful
simply refusing to answer at all
"sorry, I really don't know or don't remember..."
excessively verbose
trying to takeover the control of the interview process from you
etc.


The purpose of the devil's advocate is to make your interview process to be a failure. Repeat this type of mock interview some times if needed (possibly with some different friends filling the devil's advocate role). This way, you will be able to fix many holes in your interview process before going to the actual interviews and you will also get to be trained to handle unexpected situations.

The third step before going to the real actual interviews is to do some mock interviews with friends, teachers, your mother or something like that. Of course, you should record all your interviews (including the mock ones) and see where you are failing in order to improve your interview process and improve you checklist accordingly.

After you are convinced that no more mock interviews are needed, you could start the actual ones.