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Here are some thoughts:
1. Read through your SOP before the interview, and explicitly refer to it during the interview whenever needed. I did this and it seems to have worked out as it makes them perceive you as a person who knows his/her objectives and potentials well enough.
2. They are VERY likely to ask you to elaborate on how and why you became interested in the fields you have indicated in your application. Be prepared to very briefly but effectively develop a research question or two. This is too what I tried and seemed to have impressed them. However, be careful to develop research QUESTIONS, not HYPOTHESES! They won't expect you to be a theorist right now. I would even say it may seem a little weird if you imply this by trying to develop a hypothesis. What they expect you, IMO, is to have questions to answer which you need to do a PhD.
3. Avoid dueling on your technical abilities. Of course they appreciate your math skills, coursework, awards, etc. But what they are looking for is actually whether or not you know that research is far more than such credentials. Even if you don't have research experience, be prepared to appreciate the fact that one with the greatest possible scores will not necessarily make a good researcher.
In one of my interviews they asked me "what do you believe to be your greatest challenge in your phd education?"; My answer was "the nuances of how to conduct research". I also elaborated a little on it.
4. At the end of the interview, they will likely ask you if you have any questions. I wouldn't ask a question if you can find its answer in other resources (like their website, forums, a friend of yours who's a phd student there, etc). To ask such redundant questions is not gonna affect your chance of admission negatively. However, I suggest avoiding such questions because we all know one of their primary goals of conducting interviews is to get to know you as a person, and because this way you demonstrate that you are an organized person and you respect their time.
5. Do not think too much into what you are going to say. Do not hesitate to politely let them know if you don't agree with them on something or if you believe a question that they have asked should be redefined.
6. Good luck on your interview(s)