Depending on the school and the type of interview, it can be conducted by either a member of the admissions committee, an alum, or even a current second-year student. Different interviewers may have different expectations or qualities that they are looking for in candidates. Alumni may be more interested in your long-term goals; what you plan to do after receiving your MBA.
A current student may be looking to see if you can handle the MBA program and if you are the type of person they can see themselves going to school with, whereas a member of the admissions committee may have a more holistic approach to the interview, wanting to fill in any gaps in your application and to see what you have to bring to the table. No matter who will be scheduled to interview you, they are going to be knowledgeable about their school and MBA program.