I did the pre-screen as well. Sounds like you've got the right mindset: take everything you would expect to need for the real interview and dress for it as well. I don't think you're guaranteed to be interviewed by one specific person; it might depend on who is in the office that day. When I arrived I was asked for my resume and application so he could review them for a few minutes before coming in to talk to me. It was very laid back which eventually helped me relax. Since my interviewer was former military we talked a bit about people we may have both known at base X, and throughout our military careers. He stressed the importance of making sure everything that should be known about me was on the app; that Delta will look into everything and if you're hiding something they're going to find it. He asked me who I knew at Delta, did I fly in or drive in, discussed the future hiring prospects, and asked if I had any questions. All in all it took about 30 mins. Later that same day I had an email inviting me for the real thing.