There are potentially two, three interviews you go through. The first one is the screening interview which is general and for ALL the internships you apply for. Areas you apply for will look at that information and then decide whether they want to interview you for that position or not.
The second interview will be done by the area(s) you apply for if they decide that they want to interview you for the position.
In both interviews, you'll be asked questions on (sidenote: really, think of exceptional examples ) how well you work in a team, how you led projects in the past, how you manage a lot of work all at once, and how quickly you learn, etc. I actually wrote the first quoted passage on creating stories based on the model cast member, and it applies to the PI even more so. Think about exceptional times when you demonstrated leadership, when you demonstrated how you worked together with people to get a seemingly impossible task done, any times you might have balanced a whole bunch of work and gotten it all done, and how you had to pick up a lot of things very quickly in order to succeed at something. If you want an example, the last question was asked of me at my second interview and I used the fact that I was a Quidditch captain and coach for my university's team and had to learn how to strategize for an extremely new sport, as it was only in its second year of existence, and helped bring my team to a successful showing at the World Cup that was only two months after we had formed the team. A seemingly dumb example, sure, one that you may never think of using, but I presented it well, I was told it was extremely intruiging, and look where I am now