I'm not in HR but I'm responsible for interviewing and hiring people for my Department. Do know how the people you are interviewing with relate to the organization? For example, our 1st interview is usually with HR and they find us good candidates but really don't have much say in who we hire. Sometimes I sit in on first interviews.
The second interview is where they meet the people they will be working with daily, so we want to see if personalities mesh well and hear more detail about their experiences and how it relates to what we do. I assume you know more about the position based on your first interview so be prepared to relate your experience and come with some questions. Remember, you want to interview them, too, and make sure it is a place that you would like to be everyday! Ask more detailed questions about the type of business/clients, daily responsibilities and culture.
Good luck! Try not to be nervous and just act like you are at a cafe talking to a friend (but keep the conversation appropriate, haha). If it makes you feel better, I can definitely tell the difference between someone who is a nervous, good candidate vs. someone who isn't right for the position.
Thanks for the tips. I like the simplicity of why this company, this job, me, so thats something i can focus on and I will try to come up with some good questions for them too.