Each interview is based on past work and behaviors.

You'll need to be able to demonstrate what you as an individual did and how you did it.

In each case they will attempt to zero in on your specific contributions because they are trying to discover whether or not you're a contributor and how well you perform in both individual and team roles.

They are looking for leaders so while demonstrating expertise in your discipline is important, demonstrating an ability to both lead and follow is equally important.

I don't believe they have any difficult or unexpected questions. They are not trying to trick you. They just want to learn about your competencies based on demonstrated work in prior work/volunteer/university engagements.