There are a lot more OOS interviewed than there are IS. Each are competing for 42(?) seats respectively, for a total class size of 84, ideally split down the middle between IS and OOS.
I don't remember having to send in fall transcripts, but that could be my bad memory, since I interviewed two years ago.
The interview will be you in a room with two members of the interviewing committee, made up of faculty and staff from the school. I felt prepared because I used SDN's past interview questions portion of the forum, I'll try to link it when I get home. I also brought a small notepad to write down questions that I could ask during my interview that I thought of while touring the school. I remember that you could have a chance to sit in with Purdue's Application and Integrations class with current first or second years. I would suggest no matter what your interview time is, if you'll be in the area that morning to come in the morning because there are various presentations throughout the day. I was sad because I missed an equine presentation in the morning, but I didn't come until noon since I had an afternoon time slot.