You must be prepared to talk about the following:
- your résumé and experience
- your interest in law in general
- your interest in their law school in particular
You should probably be prepared to talk about these:
- an academic or professional accomplishment
- a mistake, failure, or weakness
- a time you worked on a team
A word about preparing and being unprepared
Don't memorize lines. That will sound stilted. This is a conversation, not an essay. Speech is supposed to be messy. If they wanted written answers to written questions, they'd have sent you written questions.
Expect curve balls. It's okay to take time to think. You could say something like: "Wow! That's an interesting question! I hadn't thought about that before. I guess if I had to give a preliminary answer, I'd say..." (But don't memorize that!)
It's also okay to say you're a little nervous: "Sorry, I'm a little nervous." Just pick up the thread. They don't expect perfection. They're watching to make sure you can perform in an interview environment without freaking out.
Questions for the interviewer
At the end of almost all interviews, they'll ask you if you have any questions for them. This is your opportunity to show your interest and ask anything you'd like to know. There are basically two categories of questions here: genuine questions and suck-up questions.
Suck-up questions are designed to show off your knowledge of a program or convey your eagerness. Example: "I see that your Immigration Rights Clinic helps clients mount detention challenges, and I'm wondering how successful those challenges have been in the current political environment." Softball suck-up questions include: "What does Harvard do to promote a sense of community in its large student body?" Make sure that the answer to your suck-up question isn't easily available online.
Genuine questions are usually more broad. Example: "what's student housing like?" I'm a fan of authenticity in general, and this sort of question shows that you're seriously considering going there and spending a lot of money on it.
A few questions are both suck-uppy and genuine. Example: "What's your favorite part of the law school?" or "What do you remember about your days as a student here?" (The latter only works, obviously, if your interviewer went there.)