Delta was the first airline I ever interviewed for this position and the f2f was overwhelming, to say the least. The room is small - low ceilings and cramped with about 100 people milling about. Obviously, trying to set a certain atmosphere to see how candidates react in that sort of environment. There were a lot of things that were the same as what others have mentioned online, so I'll try to stick to other things I've noticed.
Those recruiters were DIFFICULT to interact with. You very much have to "sell" (aka another word that starts with a W) yourself out to these people because they really came off as if they didn't want to talk to you. At all. And the mingling is absolutely dragging - imo, it took way too long in between each interview section. It's loud in there. There are at least 60 applicants (if not more) and 30(or so) employees in the joint, so you can imagine the competition and all of the hogging that went on.
Know about this company. Research them and know everything there is to know about this brand and their image. I mean it, the people that were chosen that day knew what Delta was about. Learn their missions and core values and what they stand for in this industry.
Wear the standard black or blue.
Do not be obnoxious, I don't care how outgoing you think you're being. Do not be a hog when they ask everyone if they have questions.
The people that were chosen after my interview were calm, cool, and collected. Very well-spoken whenever we had to present, sociable, but not over the top (they came off as genuine). I would strongly suggest practicing in a mirror or filming yourself beforehand.
For the actual interview itself, definitely take your time and use the notepad and pen they give you. I didn't and tried to treat it like a conversation. They are going to be stone-faced and unreadable, you might as well give them concise and clear answers. To prepare, you can find all of the questions they'd give you off the forums or wherever - they are not that surprising, tbh. All work-situated. I'd highly suggest writing your answers in STAR format on a 5x7 notecard for each question you find online and practice reciting them beforehand - I did this for other FA interviews and it helps tremendously.