I second funkybuddha917's comments. They are fairly a good summary. 

The majority of top grad school interviews, for any program, is the last filter in the application process before the University makes a decision on suitable candidates. If you have made it this far then your academic credentials and background are not in question - now it's all about your verbal communication skills and how you use them to your advantage. This is really the time to showcase your people skills. A few general pointers may help: 

1) Ensure that you convey a lot of positive enthusiasm/vibe for their program

2) Articulate the phrases in your sentences well.  And being a good listener helps in this regard.  

3) Be honest - if a technical question about your research comes up answer them truthfully, and the ones you are not sure about tell them that you will get back to them at a later point.

4) Come prepared. Do some research on the professors who may be interviewing you. If their research excites you then let them know about it. The more a professor knows about your interest and possible future collaboration on future projects the better. 

5) Treat this as any other interview and wear a business suit. How you present yourself is as important as what you say.

The best interviewees are the ones who can synthesize complex questions into simple answers in a calm and neutral expression. There is also an interview technique called PFI that some schools may use. I use this technique at work to interview new graduates. Performance Factor Interview is an interview technique used to ensure that all answers given by the interviewee are truthful. And it is easy to catch if they are lying. For example, if I ask you a behavioral question and you give me an answer I will in turn formulate a question from your answer. This continues until I am satisfied with the depth and details of your answer. For someone who is an expert in their field they would be able to weed out the liars quite easily. This is more common in corporate environment than academics however. Good luck.

-JB