It depends where you're applying to but mine was very informal when I had it last year, really just like a chat so there's no need to panic.. I didn't even sit down for the interview, haha.
One tutor looked through portfolios while another took me and another applicant on a tour of the campus, then when we got back they basically looked through the portfolio work and asked me to explain some of the pieces and asked questions such as what artists/designers influenced me and what I could bring to the course/what I thought I could learn from it.. that was basically it. The actual interview took about 15-20 minutes :]