it's definitely a little bit of both of what your “concerns” are. i'm not suggesting you act informal and unprofessional, and “casual.” I currently work with 2 of the 3 managers who interviewed me, and they definitely looked at both how I will interact with them, as well as interact with client personnel. Just be comfortable, be yourself, smile. At the interview, they'll be asking you about you and lines on your resume, and who doesn't love to talk about themselves?? Sound enthusiastic and glad that they asked you things about yourself – because you should be. Talking about yourself needs to be the easiest and most comfortable thing about your interview.
Something I often hear about candidates who didn't get an offer is that they seemed “intense.” That's pretty much all I was suggesting: be comfortable and not intense – there's nothing to be intense about.
The dinner should be easy too, anyone you run into, just ask them about themselves (just like they do to you at the interview!). Like I said, everyone loves talking about themselves, so you shouldn't run into any awkward conversations. Talk to the other candidates, especially the ones sitting next to you, and ask them where they went to school, where they grew up, etc. Don't ask them things like, what other firms did you interview with, or anything that is too personal or might require whispering. For the employees there, just ask them where they went to school, how long they've been at the firm, what they like the most about the firm, what area of town they live in, how they get to work, etc. I generally do not ask questions about sports, or tv shows, movies, etc, because the person may not watch sports, or a tv show, or a particular movie. Just ask questions that everyone has an answer to.
good luck!