It is an online skype interview, not hard, just a few normal questions about your research interest area and your plan after graduation. FYI, there is only one professor in the interview process.

I spend half a year to do research foe my second Master Thesis in Clemson with my future PhD advisor. He mad me do all the work from start to finish and would check up on me first daily, but then later only once or twice a week. After I finished all my work a couple pf weeks before I would leave, I got invited to dinner and we discussed my future. At the end he finally offered me to come back if I wanted to for my PhD.