I had a few office visits with big four firms last fall. They were different in some ways but mostly very similar.
The dinners the night before really were no sweat, though I was just as nervous as you seem to be. It was mostly associates and a few seniors. Don't worry about dazzling them. They have very little say in whether or not you get hired. As long as you stick to polite conversation, you will have no issues. Besides that, try to get to know them. Ask them what the job is like and how they interact with each other. Ask them about their best and worst days at work. Get answers to the questions that will help you decide among firms if you get multiple offers.
The office visits were identical across all firms. You begin the day in a conference room, where you'll be greeted by the recruiter, maybe some breakfast, and always an assortment of branded tchotchkes (pens, post-its, thumb drives; the firm I chose gave us all backpacks). After you've checked out the goodies, you'll get the spiel about [insert company name here], usually from a partner. They'll you about the history of the company and some of their bigger clients. After that, you'll have 2-3 interviews with managers and partners.
In contrast to the first round of interviews, they will spend some time trying to sell their firm to you. However, they still are evaluating you. While everyone who gets an office visit has a good enough resume for the job, they still will have more candidates than open positions, so they have to narrow down the field. Express your strong interest in the job, and highlight the reasons why you think you'd be a good fit with the firm. Ask some questions as well. Some of the questions I asked opened up some very interesting and enlightening conversation pathways that helped them get to know me better and helped me choose among offers when the time came. At the end, make sure to get business cards from your interviewer.
At some point, you'll go to lunch with some more associates and seniors. Once again, they have little say in whether or not you get hired. They are there for your benefit, to answer your questions. You also will get a tour of the office at some point during the day. If you have interviews left after lunch, you'll complete those. If not, you'll be shown out of the office, and you'll await their decision. The amount of time you have to wait can vary. The firm I chose called me less than an hour after I left to offer me the job. I received an offer from one firm a week after they interviewed me. Make sure you follow up within a few days with at least one of the people with whom you interviewed. Reiterate your interest, and make a final pitch. After that, just try to stay calm.
Congrats on your second interview. I hope this post makes you feel a little less nervous and helps you prepare for the process.
If your nerves haven't been soothed, here's a story that demonstrates just how hard it is to screw up. I was in an interview with a partner, and I asked him about his path from associate to partner. He told me about his career progression and the different offices where he had worked. Without thinking, I asked him to tell me about the Chicago office because I did my first undergrad nearby and wanted to live there at some point. Keep in mind I was interviewing for a position in a medium-size southeastern market, about 1000 miles away from Chicago. I basically told him that I wasn't long for his office if I got the job. I immediately realized my mistake and tried to correct the error. He interrupted my attempts to walk back my comment and said, “Huskies1026, [my city] is not a growing city. It's a going city. I completely understand your desire to see other, bigger places. I felt the same way when I was your age.” I was somewhat relieved but still worried that I had torpedoed my chances. An hour after I left the office, he called me to offer me the job. A week later, I called him to accept. When I told him, he jokingly responded, “What took you so long? Trying to get a better offer in Chicago?” We laughed, and he continued, “Huskies1026, we're glad to have you on board. We hope you'll be with us for a long time, but if there comes a time when you want something different, just let us know and we'll do what we can to make that happen.” Even if you make a mistake in the interview, it could work out.
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