Whether or not they ask Why Harvard, during the conversation, they will want to see that you have well considered reasons. Plus confidence, maturity and the other attributes they look for.
In addition, be prepared for a question interviews are not supposed to ask, but often do:
What other colleges have you applied to?
This is especially true in the RD round, where an alumni interviewer may awkwardly ask:
Did you apply to any colleges in the early round?
Were you accepted early to another college?
Which college is your first choice school and why?
Also, I think it’s helpful to look through sample alumni interview reports, so you can read what kinds of things alumni say. Harvard doesn’t have sample reports available on-line, but Yale does. Play special attention to the interview reports for JEROD, RICHARD, and THERESA: http://asc.yale.edu/samplereports
I personally think it is unnecessary to do this kind of preparation. Just settle your nerves and understand that you need to choose Harvard, as well as the other way around, so think about fit. And be yourself.
In some ways I think spending too much time studying and preparing for an interview is counterproductive, kind of like an over-edited essay. Just be natural. Go to the movies the night before.
If you don’t have good reasons to go to Harvard, don’t apply. If you do have good reasons, state them if it comes up.
Interviewers tend to be nice people. Just talk to them. Relax.