Congrats on the interview! I had my interview a few months back, but this is what I remember. The overall experience was very relaxed. Each of the interviewers had a list of questions. My student interviewer barely wrote in it. My faculty interviewer did write a lot though as we were talking.
I had my student interview first. He was really nice, and we just ended up having a conversation. He didn't asked me anything tricky. I suggest you research some interesting sites in Boston that you would like to visit. That was the hardest part in my interview because I didn't have specific locations/sites when he asked me what I planned to do if I were to be in Boston the next four years. Other than that it was really relaxed.
My faculty interview was second. She didn't ask me anything tricky at all. Most of the questiosn that she had asked were expected. Some questions did required specific examples (ex. What classes did you like and dislike in undergrad?) She asked about my extracurriculars.
If anything, just remember to be yourself. remember to answer the question. be specific in your timeline. use specific examples (no vague answers or general answers).