The two interviewers asked a standard set of behavioral interview questions and kept taking notes. Other than that, I had lots of opportunities for Q&A (before and after the standard question set), and I kept thinking hard about BJS-specific questions to ask.
If the applicant is not in the DC area, the agency allows the interview to be either on-site or over the phone. I happened to be in that area for a conference the day before, so I went there for an on-site interview.