Recognize that interviews done by alumni are going to vary widely, although the colleges do try to tell interviewers what they want.
My husband and I (we're both MIT alums, so we do the interviews in tandem whenever we can) typically spend 1-2 hours with a kid, usually at a restaurant over a meal (that helps the kids who are shy and puts some structure on the time). We don't stop the interview early if the kid isn't done. We like it when kids ask us questions (in fact, it's a bad sign if they don't have any curiosity), but it's best if they have already clearly done their homework -- don't ask easy things like, "What kinds of engineering degrees does MIT offer?" Mostly kids ask us more about the life of a student there. We're not going to quiz a kid on their knowledge of their field, although we will give them an opportunity to show us what they know if they want to. If they know a lot in a field, we'll ask them to think a bit creatively or speculatively within it. But the fact is, we want to know more about their working style, how they handle failure, how they handle uncertainty, how they pursue their passions, how they work with others, etc.
There's no gotcha here, and even in those interviews where we do feel that this kid is not going to be a good fit, we work extra-hard to try to give them (1) the opportunity to show us their best side (2) the feeling that they have been treated respectfully and thoughtfully and kindly. In general, we don't tell them if we're not going to recommend them for admission -- that's just mean.
I can think of one time where we told a kid that he was not likely to be a good candidate, but that's because he had a very unusual and checkered academic history that didn't yet have the "uphill trend" at the end that would show an admissions committee that he was going to be okay. So we talked with him about what he could realistically do with the next year or two that would help him establish a solid record of follow-through on his goals, so that he could potentially be a better candidate in the future. He seemed to appreciate it.
