Why do "everyone suck at their first research interviews"? I needed to do some research interviews for my master thesis. I study software engineering and in the interviews I ask people about preferences and more regarding a piece of software. 

When listening to the recordings it's obvious that I did a very poor job in so many different ways:


ehm, ehm, ehmmmmm, ehm, constantly
interrupting the interviewee
forgetting to acknowledge his answers
missing important questions, though they are in an interview guide
failing to formulate my questions clearly
elaborate on questions way too soon, because I find the silence uncomfortable
worried about the interviewee feeling that he is wasting his time, or that I am stupid or incompetent
sounding nervous, yes I was
not sounding interested in their answers, though I certainly was
my speech gets sluggish and less precise than normal, I really just sound stupid (more than I am)


...and much more.

I spoke with two different anthropologists that I happen to know. They said that they had the exact same experience, and for the first many interviews they tried. They say there is a consensus in their school about how it takes time to learn, as a craft. According to them one can prepare very well, which will help, but still the interviews will be quite bad, but that in time one will learn and get much better. Even the professors said this, that they should expect to do bad at first, but later they would learn, that this was almost inevitable. 

This was nice to hear for me :)

I need to reflect on this in my thesis, that my performance was so bad. To do so, I need a source. 

My questions is: If there is a consensus about this, are there some sources supporting this claim, or just sources dealing with this issue?
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.