That person will role play questions you can expect for the kind of real interview you will have very soon. You will respond to the questions as if it is the real interview. The feedback is designed to provide you with an objective measure of your level of interviewing performance. For example, maybe you are not making enough eye contact or you might wiggle your foot when you answer the questions. Everyone can improve upon their interviewing skills in some way.
Many strange behaviors manifest themselves out of nervousness.
After you become more confident, most of one's nervous gestures begin to disappear. Most importantly, the mock interviewer can critique your responses to the interview questions. They can also provide you with a full critique of your overall appearance if you choose to mock interview in your interview suit. You may set up an appointment any time with the Graduate Assistant who has taken great effort to be trained to conduct mock interviews and provide constructive feedback. Appointments are typically about 50 minutes with 20 minutes for the mock interview.