At the risk of sounding a tad bit generic, I would encourage you to relax and just be yourself in the interview. Many people make errors because they try too hard to be what they think the admission's committee wants them to be or to say what they think the admission's committee wants them to say. Remember, you are interviewing them too, so the power does not rest entirely with them. If you have ever watched the show Shark Tank, you know that once the entrepreneur has made a compelling case for their product the Sharks have to compete for the affection of the individual who came on the show. If you are at the interview stage, they are already interested in your "product" - this is an opportunity for them to buy into you as a person. Also, they have a reason to sell themselves too. If you let them think they did a compelling job at doing that, their perception of you will be higher (basic psychology - we like to be liked, and we like those who like us).
@Almaqah Thwn is definitely on to something with his advice about brushing up on the details of the department's website, and I would add that it can help to have a few well-formulated questions to ask them about their program that is not immediately available on the website (it helps you sound interested - see the point above). It can also be a nice touch to send a very brief email thanking the professors for interviewing you, but do not make it desperate or forced.
