Lunch with accounting firm I may be interning for TIPS PLEASE

I am having lunch with BKD on Monday at Pappas Burgers. I am N-E-R-V-O-U-S.

First of all, what do I WEAR? Another guy from my school is wearing a suit but he wears suits everywhere for some reason. My initial interview required a suit. The dress code for BKD, I imagine, is suit stuff. But it is just Pappas Burgers and they made it sound more like a social than anything. Do I need to wear a suit or will slacks/dress shirt be OK?

Second, what do I SAY? What kind of questions do I ask?