I'm a 3L who went through numerous callbacks last year. Jacy's advice was great. I'd also add that you should take a quick survey of the attorney's office as you walk in and look for anything that broadcasts that you have a similar interest (photos of kids or dogs, a softball trophy, golf clubs, you get the idea)--these make great fodder for kicking off an interview and making small talk.
Also, don't worry if your interview consists more of get-to-know-you questions than law-related questions. If you hit it off with someone and they don't ask you their pre-written questions, it's probably better that way. A good interview should be more of a conversation and less of a question and answer session.