No need to panic. Grad school interviews are almost formalities (I'm talking about Biology programs). If you have been invited, they want you, or else they won't pay for flight and hotel expenses and treat you like son-in-law. Having said that, it's absolutely essential that you know what your application essay or resume contained in terms of your research experience. Some professors can be tricky and some might ask you a lot of questions because they are interested in what you did. Be proactive. Learn about that professor ahead of the meeting, get a sense of his/her work and ask questions. Honesty is the best policy; if you have no familiarity with his/her line of work, politely ask him/her about it, and on the other hand, if you have some background, be prepared to ask related questions or provide input. Almost all the schools will have a student host assigned for you. Express your interest in attending that school to your host. Student input counts in grad school admissions. One purpose of these interview/recruitment weekends is to weed out troublesome/socially inept students. Act normal and show interest, you will be fine. Let me know if you have more/specific questions.