First, remember that you were invited! Clearly, the interviewer liked what they saw on the resume and your experience and qualifications got you this invitation. Typically, when working with an agency with a recruiter, the interviewer already knows you can do the job. The interviewer is probably most interested in you — your character, your temperament, your ability to fit in with their existing team, your ability to produce quality work in a high-stress environment.
Second, an interview is an exchange of information. Not only are you there to share information and talk about your employment history, you are there to listen. Often, listening will provide important clues — from what the interview says directly, or from what the department’s attorneys are seeking — that will help you land the job.
Make certain sure you clearly understand any question and that you respond directly and without evasion. That does not mean you are supposed to tell the interviewer what you think they want to hear. No one expects you to be “perfect”; in fact, prospective employers prefer for you come across as “human” and have a humble approach to what you do and do not know. Responding appropriately is the nuanced difference between landing the role or not. Evading challenging questions is certain to damage your credibility.
Third, please come to a job interview prepared. As much as you want to land a job, you actually want a job that is a good fit for you, allows you to convey the experience and skills you possess, and works for your personality. Law firms are as different as people: some are relaxed and easy-going; others are tight as a bow string. Ask important thoughtful questions so that you and the firm can decide mutually if this job would be a good fit for you.
You would not have been offered an interview unless you were a rock star. Take a deep breath, square your shoulders and share your best, most confident self with the interviewer.