<p>Personally if this just happened I’d draft a nice, short note to the person who interviewed you first thanking them and then asking whether it would be possible for you to speak with them again (given whatever reasons…) and how you’d appreciate making the most of this opportunity to get more familiar with Harvard, or whatever.</p>
<p>Make sure the email is crisp. Also, maybe in the subject line indicate that you have a concern, with something like “Thanks much for the interview… important question” because I’m sure he/she is used to getting the cookie-cutter thank you note and he might not even read your email.</p>
<p>Again, this is just what I would do on a hunch, you might be better off contacting admissions (see what others have to say). Even if you performed poorly, you get the sense of how the interviewer is, based on which you may frame your email appropriately.</p>