My advice would be to definitely use the interview to find out more about the school. You'll be talking to someone who actually attended- and this is a valuable resource in deciding whether you want to attend in the end if you're accepted.
If they are an older interviewer- (one of mine was class of '69!) you can ask them about how they think the school has changed over the years, or what they would recommend someone in your shoes do with their college experience. If they're younger/just graduated (another of mine had literally just done so) you can ask about what doors the school opened up for them, how good the careers/academic guidance was, whether the alumni network has been any help to them, what their fellow classmates are up to... things like this help you get a feel of what it will be like for you if you were to attend their school. It's always good to ask what they enjoyed most or what they would have liked to change about their time there or the school itself.
Remember, it's a two way interview. Yes, they want to know if you're a good fit for their school but you want to know if they are a good fit for you, too.
And, as said above, it's a casual conversation and not an interrogation. It doesn't have much weight in the admissions process so have fun with it.