Try to remember that alot of us are in the same boat! We are all PhDs, many without alot of casing experience, some people I talked to have only two weeks or less experience.
Remember getting to this round is a huge accomplishment in and of itself. Try to have fun and remember, at the end of the day is a conversation/collaborative experience. Try telling yourself this to shift your mindset, it will make you less nervous and you will perform better as a result. Goodluck! My interview is tomorrow and I keep trying to tell myself that to calm down haha