5 min skype in English, not tough, simply went over the previous research experience and what I am interested in doing in the future. Just be prepared and willing to communicate, you will ace it.

If you are invited for an interview, you will most likely have interviews with 5 professors (either you chose previously depending on your interests or chosen by the school). These interviews are semi informal. Apart from these interviews, you will also have social time with current students or faculty members to learn about the school, program, etc.