There is none. You just need to simply push yourself until you get to the end. The interview is your time in service. Your ability the pick up and keep rank.

No real interview process. Be a great Marine. Show your motivation and initiative. Show case your leadership abilities. Be great at PFT. It pays to network and be liked by your seniors. There are Marines I served my whole five years with. I made it to Sergeant, and became an Infantry Squad Leader. They made it to Lance Corporal, and never placed in leadership position. Why? They failed the interview process...