Interview with the department head, how should I prepare?

I was referred by my friend for a summer intern at a hedge fund. She is in the same department and after she sent my resume to HR, HR emailed me to do a phone screen with the department head. I’m very nervous because I initially expected to have phone screen with HR but turned out to be the department head. My friend knows him as well. But I have never spoken with a such high level employee before. Can anyone provide some insights on how to prepare?

I really take it very seriously not only because I really want this opportunity, but also don’t want to affect my friend’s reputation because she is the one referred me and they know each other.