You are being interviewed to see if you have the technical skills to do the job, and to see how you well you interact with other people. You must learn to be confident and friendly. These are foundational skills.
So I guess you're saying I should be studying people skills, I guess? I often hear that I need to get mock interviews but there are few official sources for it. Also it's not very instructive to tell someone to just find any friend. Most people do not have experience interviewing programmers, so they wouldn't be good for mock interviews. And mock interviewing with friends would introduce too much of a positive bias you won't have from strangers.