Most of my interviews were very positive experiences, and there were no "hard" or tricky questions. Be prepared to discuss your research interests in detail, as well as any projects you are currently working on, and to talk about how all that will fit in with what your potential advisor is working on. Be prepared to ask a LOT of questions. I had one interview where the entire thing was a prof asking, "So, what can I tell you?" over and over again as I asked questions which he answered. He never asked me a single question (besides that) but was obviously getting a feel for me based on what I was asking. Having questions to ask is as important (if not more important) than having answers to questions.
Primarily, be personable. Smile. Act like a person that would not be excruciating to be around on a daily basis for the next five years. They have already decided that you're smart enough-- now they're trying to decide whether or not you're tolerable.