The PBL group interview is a mock PBL case. You will not be asked any questions during the interview process at USC. Despite what some of the interview feedback might say on SDN, the admissions committee is looking for individuals that express the ability to work well in a group. This does not mean being "social." They want an individual that can contribute to the group in a MEANINGFUL way. Just talking is not going to get you anywhere. Additionally, they want to see you give and take feedback/criticism. Other members of your PBL group might have something good or bad to say and they want individuals that can identify these qualities and state something constructive. Most importantly though, you cannot make yourself be/do these things. You will look foolish. It is better to go into the situation and be yourself. It will probably the least and most stressful interview you have. By least stressful, I mean you will not be put on the hot seat at any time; however, the stressful part is not knowing if YOU are what they are looking for in an applicant. Good luck.
When is your interview?