Every state differs. Every representative differs too. I can share my 1 experience so far with my state interview.
Mine was 3 people interviewing me. There were 3 rooms (and I assume 3 people in each room) where 3 people were interviewed for 20 minutes at a time. Mine went 30 minutes because they kept asking very specific questions on my application that I can't disclose without doxxing myself.
They first asked me to introduce myself, why I want to attend a SA, and since I'm applying for 3 SA they asked why I want to go to 3 SA and why I ranked them the way I did. Since I had some "special circumstances" they asked me some tough questions about that. Honestly, it was pretty different from my ALO/BGO/FFR interviews because they didn't ask questions like "are you prepared to do x" and more like "why do you want to do x".
I didn't practice much. I rehearsed the generic questions like why I wanted to go and some things about me I wanted them to know but I didn't want the interview to feel scripted. I also practiced not saying "uh" and "like" and what I was going to say when the interview ended.
Important things to highlight would be your leadership and what makes you YOU. Everyone at that interview has similar stats (GPA, SAT, captain, etc.) so they want to know what makes you unique and better than everyone else. Make sure you 100% want to go to a SA because you won't convince them that you want to go if you can't convice yourself.
Dress nice. You won't overdress. If you're a guy wear a full suit (tie included) with the coat and nice shoes. You'll feel professional if you look professional. If you're a girl wear a nice dress (or a suit) or something that you would wear to the most important job interview of your life.
Make sure to be early. I was going to be 30 minutes early but with traffic it was only 15 minutes. It's better to wait than be late. You also don't have to bring your parents. I didn't and I only saw 1 other candidate with a parent.
If you have an LOA don't forget to send it. The interview will go a completely different route if you have an LOA.
Don't forget to take a deep breath and relax. It's more of a conversation than an interview. Be professional but not scared. Clearly you are a very strong candidate for the nomination otherwise you wouldn't be there so don't panic.
Oh and remember to keep the little nametag they give you. My mom wanted it for scrapbooking so keep it for their sake
