I had interview with HBS this week. I was also intimidated and got stressed out, but it ended up fine. They asked generic questions - why phd, why this field, research interests, etc. In fact, it actually seemed like that the school had already made mind whom they would admit. The interview felt like out of formality and maybe make sure that people are not weirdos or unable to carry small talks. So stay chill and be yourself! Hope we can meet as a cohort member.