I was also a hospital volunteer and I'm surprised that you didn't have to do much. What exactly did you do there? Did you not have to help deliver medicines to different floors? They even took me to the crazy area once, which was kinda scary. How long have you been there? They didn't know what to do with me on the first day but 30 minutes later I was working nonstop. Maybe you're lucky and your hospital isn't that busy. If any of the tech seem busy ask them if you can help.
But back to the topic. From what I remembered of my interview, they asked me to elaborate more on my experience at the hospital and I just told them the general things I did and what I got out of it. There was only ONE really tough question that was asked by a student but it wasn't pharmacy experience related. It was a lot more like a normal conversation. Don't fret too much about the interview. If you've received an invitation then you're already 75% of the way there.
