-- you are correct, E, that many interviewers in call backs have been told to assume that once screened and invited back, the candidate meets the firm's cutoffs for grades, etc. Some interviewers are nevertheless grade sticklers and may question you about grades. If the grades weren't exactly at top and outside the cutoff a little, I would expect more "wowing" from the candidate.
-- what else is different other than the interviews being longer? In some firms, behavioral interviewing is implemented. That means that each attorney who sees you is assigned a particular attribute to observe and evaluate, e.g., "judgment," "maturity," "interest in firm." They will ask you questions that try to get to these issues. At other firms, these items are standard on the evaluation forms.
-- lunch: typically you will have a lunch with a couple of attorneys. I have said this before, but your lunch conversation, discussion over the meal, and overall demeanor is crucial. Do not let your guard down just because you assume the interview is only over or just because you may be with some younger (junior) attorneys. As HP, if I have a particular issue of concern that I want explored, I will frequently go to the attorneys who will accompany the candidate to lunch and ask them to see if they can judge this issue for me, since they have more time with the candidate.
-- people: since it is a longer day, you will see more people. Hopefully you will meet with men and women attorneys at various points in their careers. Each brings a different background and view of candidates. It is more difficult to impress 6-8 people than the screening interviewer. It is unusual for one candidate to WOW all attorneys on the call back schedule. The goal is not to offend or annoy any of them. If you get a list of names in advance, read their bios, and even Google them. Especially when you have a more reserved interviewer, it is helpful to have some questions that draw on their area of practice, recent experiences etc. If a lawyer's bio says nothing about pro bono or community activities, don't send those questions his or her way.
-- look around. You are a buyer as well as a seller. See what is going on in the office. Quiet? Pleasant seeming? Doors closed? Open? Staff friendly? This is your opportunity to observe as well as to be observed. HP once walked into a very white shoe NYC law firm for an interview and it was colder than Siberia. HP knew this was not the place for HP long term.