I know stress levels run high during interviews and we often try to say the right thing, and/or worry we're going to do something wrong...but here are a few things to keep in mind while interviewing:
1. Your application impressed them and they want to get to know you. If you try to answer like someone else would..or you try to be anyone other than yourself (i.e., taking control or trying to be domineering if this isn't your normal mode of operation), you run the risk of seeming pushy/unprofessional/silly/immature/scatter-brained/uber stressed. Be yourself and trust in who you are. It's what got you this far. You deserve to be there or you wouldn't have gotten the invitation.
2. The people that are interviewing you are experts in their field. Show them the respect they deserve...they've also done this dozens of times so they know what they are looking for in a grad student. They are not the enemy nor are they a TV journalist trying to catch you in a slip-up. Do your homework (know their research, your research..etc) and you'll do fine. You are sitting in that interview for a reason...you obviously have what it takes to do this.
3. If they are trying to stress you out, then they probably know exactly what to do/say to get a reaction from you. It's OK to not be perfect. They want to know how you are going to react under stress so just act natural. If there is a question you do not know how to respond to, tell them that their question is one you haven't considered and would like a moment to think about it. This isn't a game show...there is no clock ticking down the time remaining before you lose the $64 million question. If you want to control the situation, then retain control of yourself.
You can do this and you will rock it. That's why they wanted to meet you! 🙂
AB