I thought I would write up a bit about the interviews I did in the fall since a lot of people have been asking about them so hopefully this helps you and a few others. I was really nervous before the two that I did, but in hindsight, now feel that they went really well. I haven't heard back from any of the schools I've applied to yet, so I know for some that may seem like I don't have the evidence to prove how well they went, but the point is that I feel like I did the best I can which is all we can ever hope for.
I felt at first like I needed to read all of the work by the faculty members I would be meeting with before the interview, but this is just not realistic. I'd say, think about why you want to work with that person, and then read one (maybe two) major papers by that person. If their major text is a book, read book reviews, and at tops a few chapters. Don't overdo it - it's unnecessary. Believe me, I was the person saying to myself, 'Sure, sure, OTHER people may not read the collected works of the faculty of each school their applying to, but I WILL.' I didn't. I'm an overachiever, and I didn't. Be compassionate with yourself. Have enough to be familiar with their research and interests, and so that you can ask them questions about where their research is headed. This will also help you in understanding the agenda of the department, or of the research center you're applying to, if that is pertinent. If the program does have an agenda, like Yale's Center for Cultural Sociology does with the Strong Program, then read up on that mission. If you know any grad students, talk to them first about the program and the faculty and their interests, because they'll be able to give you the most updated info sometimes. This happened for me at Northwestern - one faculty member I was really interested in, it turns out, might retire soon and the focus of their research is not really in line with the work they've done in the past that excited me - and I gleaned that all from a current PhD student.
Review your own research interests, and if you have it, your thesis or major papers. If you haven't written any major papers, use your writing sample. Think about the questions that really interest you, and practice giving an "elevator pitch" describing those questions and your vision for yourself. If possible, show how you're aware of the questions place in the larger scheme of Sociology and the department itself.
In the interviews that I did, there was a wide variety to how the conversations went. I met with 7 faculty members total for two school visits, and one of the interviews was really short (only 20 minutes) and I felt like I had to be really quick on my feet to answer questions. Another one lasted two hours, and ended with the faculty member giving me a tour of campus. I'd say, give yourself some flexibility when making appts with faculty if you're visiting the campus, and be clear with them about your expectations for time at the beginning so that they have the opportunity to either agree or tell you that it'll be longer or shorter. That way, you can gauge how much time you have, and if you have less than you desired, you can get right to the heart of your goals and interests right away.
Most of the faculty members just wanted to know more about me, why I have two BA's, why I landed on the research questions I did, how I came up with my thesis question, if I had any statistical research experience. No one asked about GREs or GPA. So, that's why it is essential to practice your "elevator pitch" before you go. Really get to the heart of what you want to do. No one asked me my opinion on Weber or positivism or interpretive sociology... no. Some theory came up naturally as I explained my passions, but it was not a quiz about my intellect. They know we don't know everything - that's why we're applying for a PhD! They want to mold our minds - we need to show we are at least somewhat malleable. And then ask them about what they plan to do, too. What's up next for them? How might you be able to work with them on those projects? Also, don't be afraid to stand up for your own opinions. I had two professors challenge the generalizability of a quant paper I wrote and my research design, and I was very candid and forward about my views on generalizability and my goal for the paper. I defended my work and my beliefs, and they're looking for that.
If you're visiting campus (and haven't been offered admission yet - and if your admission would depend on your visit), and sitting in on a class or a workshop, ask the professor beforehand if you are allowed to participate in the conversation. If possible, get a copy of that day's reading before and know your stuff. Ask questions, participate in the conversation, offer your opinion. This will give them an opportunity to really see your mind work.
This was a bit of a stream of consciousness. I hope it helps! It's hard to give good advice without knowing everyone and their specific cases, but hopefully this helps a bit