Congrats on getting the interview!! I interviewed with my current boss three years ago and I had the same feelings as you did. He looked super serious and asked hard questions that I didn't know the answers to. I left the interview feeling bad. However, I still got the job, and after working with him for 3 years, I gotta say he's the best boss ever! He never appeared to be that dead serious anymore so I guess he happened to have a rough day when he interviewed me or it was me who was so nervous that everything appeared serious from my perspective, which is not necessarily the reality.
I also had a "bad" interview last year for a DrPH program during which the interviewers were playing hardball but I still got an offer!
I hope my experiences make you feel better, and I can't wait to hear good news from you!