It varies by school. At some schools, almost everyone who gets an interview eventually gets accepted, and they're mostly trying to make sure you're not completely weird/inappropriate, and also trying to sell the school to you. At others, they want to make sure you can talk about your research intelligently and ask good questions. If you know who you're interviewing with, maybe do a little prep by looking at their lab website to familiarize yourself with their work and maybe read a paper or two, although in my experience that hasn't helped that much. Just be confident, understand your research experience, and be genuinely interested in what they do and the program.