If you want an idea of the type of questions or preparations you should do for a civil service position, try this link
http://www.usajobs.gov/infocenter/interviewtips.asp . This site is for federal government positions; however, many state and municipal civil service positions follow the same format. It's a short list, but has some good points. The federal Office of Personnel Management has other job tips. Do a search for additional interview tips and possibly a list of interview questions similar to those you will face!
I served on interview boards for fire department positions (entry and management level) in several states. Here are some tips you may not find on some tip lists:
1) Wear a suit! You will be viewed by the public as the "police department"; show your potential employer how well you look and how you will exemplify the department while on patrol, meeting the citizens, and while testifying at trial.
2) Clip your fingernails. I know, sounds weird, but the first part of your body with an up close view is your hand when offered to the interviewing panel. You should look neat and well kept.
3) Shake hands with each interviewer and have a firm, but not overpowering handshake. You want a job as an officer, not as a doorman. A firm handshake demonstrates confidence; this handshake tip applies whether male or female!!
4) If interviewed by a panel, make eye contact with each panel member while responding to the interview question. This will make each member feel like you are responding to the member as an individual; also, this demonstrates to the panel your ability to speak to several persons as once and shows you can "have a handle on things" under pressure.
5) Practice some of the things you learn BEFORE the interview. If possible, have someone ask you fake interview questions and video record your session.
Remember, many people applied for the position and you are one of the few with a chance to get the job. Don't lose out because you weren't prepared. If you want it, I have some more info for you; PM me if you need it!