Hey dgh44 - CONGRATULATIONS!!!
Getting the interview invite is a REALLY BIG DEAL! This is an important step in the process. Nice job!
The actual interview experience itself is somewhat different based on which school we're talking about - we're gonna guess that this is Columbia, since they're the only ones doing much interviewing right now. Most schools, Columbia included, do "blind" interviews, where the interviewer will only have access to your resume, and will not have read the entire application or the essays (MIT, Harvard, NYU are among the schools that do it differently).
The interview is typically 30 minutes, and generally focuses on the main questions of why do you want an MBA (career goals) and why do you want to go to that school. Essentially, the same information you covered in your essays will be asked in the interview -- and for a "blind" interview like Columbia, it's totally fine to use the same material and same stories that you presented in those essays. Obviously you want to be consistent with everything you say here and what you already told the adcom in writing before! And, it's also a chance to add more details or include other stories about accomplishments and leadership that you couldn't fit into the essays.
First and foremost though - be enthusiastic! Show them why you want to go there based on detailed answers to questions, with specific references to classes/clubs/etc that that school offers. This is one important way that you can convince the interviewer that you're really motivated about what they have.
Generally interviews are friendly - this will not be an adversarial experience, they're not trying to trip you up. They're often just as interested in selling YOU on their school as they are in seeing what kind of person that you are and what type of contribution you'd be able to make on campus.
So, practice your answers - go through the standard questions about why MBA/career goals etc with a friend - dress professionally - be prompt, friendly, and polite - and you'll probably do great!
If you want to share which school it is, we can offer more specifics.
We hope to hear back about how it goes for you, too!!
Good luck!!
EssaySnark