Part of the interviewer's job is to provide you with information and your giving the appearance of knowing everything already might be off-putting. Be aware that government position descriptions may not really describe the job, just give you an overview. The PD might not give you the answer to that one and the interviewer will like the question. It is natural to be nervous in anticipation of an interview. Remember, though, it is a meeting of those with a mutual interest.
Interviewers are sometimes stern without intending to be.
You are there at their invitation. A pleasant, engaging manner on your part will make it easier for the interviewer. If you feel like your interviewer isn't doing a good job, help them out by talking a little more yourself and asking a few more questions. Maintain eye contact and be certain that your body language is natural and complementary to the discussion. Be prepared to talk about yourself.