Interviews for PhD programs can take many forms. In my case, since I was an international applicant, the interview was to be conducted using Skype. So, if that's your case too, be sure to have what you need for a Skype interview (webcam, mike, good internet connection etc) and test everything in advance. Also be aware of the difference in time zones, since there are several different time zones used across the US. An interview for a PhD is also a lot like an interview for a job. So, check the way you dress and talk, for example.
On February 16th, exactly 8:30AM US central time, I receive a Skype call. The professor presents himself and tells me about how the interview will be structured. A few questions, more like a chat, and the opportunity for me to ask a few questions in the end too.
He started asking me to present myself. Then, several typical questions in PhD interviews, like "Why do you want to do a PhD and why now?", "What are your plans after the PhD?", "What are your research interests?", "Why the interest in the University of Houston?". You can see that the questions I was asked is a lot like the questions we think about during the Statement of Purpose. Since the Statement of Purpose is very concise, the interview is a chance to explain a few aspects of your application, and add depth to what you wrote at your statement of purpose. Take that opportunity to talk about things you wanted to write at your statement of purpose, but left out for lack of space.
I think I gained a few important points in my answer for the question "Why the interest in the University of Houston?" in particular. Because I had a lot of information about the University of Houston, Bauer, the faculty I was interested, the research they conducted, and the city of Houston. So, lots of reasons to show why University of Houston instead of other universities.
There may be also some questions which are not so typical of PhD interviews in general, but more specific to your concentration and your profile. Since I was applying to a PhD in Quantitative Marketing, I was also asked about my knowledge and skills regarding Economics, Programming, and Quantitative Research. Since my background is mostly in Finance, I think Economics is not a problem in my profile. I also had some recent quantitative research done, during my master's. But I have very little experience with programming. I thing the last time I programmed something was when I was a teenager. Be honest about your weaknesses. Show that you are aware of them, that you've taken steps in dealing with them. I had already started to read about programming and that helped, even if the kind of programming I was studying was not the one they wanted. If the person who is interviewing you know your weaknesses, he/she can talk about how the school can help you. And that's a great topic for conversation.
The interview ended with a few questions of mine. I asked the professor to tell me about his style when advising students, what kind of research he intends to do in the future, thinmgs like that. So, questions to allow me to have a better idea about what should I expect if he was to become my advisor, for example. I also asked a few questions about the opportunities for teaching and research assistantship and about campus housing. I actually said a lot more questions than the ones I asked. But since the interview had already been taking a long time, I said I would have additional questions if I was accepted. And yeah, after I got my offer, we had another Skype talk when we discussed a lot of things I wanted to know.
In the end of the interview, the professor said I was expected to have a decision in a few weeks. So, I just was back into waiting mode.
On February 21st, I received another e-mail from another professor from Bauer, asking for another interview. And that interview would be on the same day. Yeah, I got the e-mail in the morning and the interview was held that same day at 2:30PM. I think this shows that you have to be always ready, because you may not have much time between an interview invitation and the interview itself.
This time, it was a professor that I didn't know (not one of those in my Statement of Purpose). His research interests also did not seem to be such a good match to my own interests. So, this time I would go to the interview feeling a lot less prepared. But a second interview could only be a good sign, so i was really happy and excited.
This professor said that it is a standard procedure at the University of Houston to take two different professors to interview PhD applicants they are interested in. Questions were about the same of the first interview, but the style was a little different since professors have different personalities. In the first interview, the professor asked a question and usually just listened quietly to my answers, sometimes adding something. In the second interview, it felt more like a conversation or negotiation, with we both debating about aspects of my application. It felt more like he wanted to know if I knew how to argue against something he said. It does make a lot of sense, if you know he is an expert in sales. I had to sell myself.