Initially I spoke with the head of the division over the phone. At the time I was in graduate school at Georgia Tech, and he had gotten a copy of my resume from one of the career events (either one of the on-campus events, or one of the GTRI-specific events, which I would encourage you to attend if you are a US citizen and a student at GT).
After that invited me to their building. I came by a few days later, in a suit a tie. I spoke two or three branch managers, and a senior engineer who would later be my direct supervisor for a period of about a year. They did most of the talking. It was mostly informal, and honestly I could barely follow what they were talking about. I was a strong student, but it was a big rush and the subjects were only somewhat familiar.
I also spoke with a couple PhDs (most of them are not managers, but have similar seniority). This was largely informal as well. All in all it was a lot of fun, but I can't say my experience would be representative of yours. I was coming from the same institution, and already had some idea of what they did. GTRI is a very heterogeneous place, and people do things differently all over. Your interview could end up being very technical, so buyer beware.