Relax.  It is a friendly, freeflowing 1 hour job interview, just like the kind you expect in the private sector.  You should ask the person in the Recuitment Center that you're dealing with what office is interviewing you, if you do not already know.  Chances are, you have something in your past that relates to what they focus on; you will spend lots of time talking about that.

Come with questions, but also have a good understanding of the agency and what role you would be playing.