I agree with DG and, for the most part, CS above. However, if you do send a follow-up e-mail to expound on something that you think you flubbed in the interview, keep it short. Whether it is a second-year student or an adcom member, your interviewer is
very busy; if you write them a novel I think you are going to hurt yourself more than help.
Also, interviewers know that it is much easier to sit behind your computer with unlimited time and craft an answer to a question than it is to answer that question face-to-face in an interview situation. My point being: don't expect a follow-up e-mail with a better response to save you from a bad interview.
The interview, from what I can tell, is to gauge whether 1) you'd be a good classmate and 2) you'll be able to interview well and get a job after school. They already know everything relevant about you from your application, so they're not necessarily looking for new information. They want to see how you interact with people.