Hey! I just had my interview at IU, and I don't know the outcome yet or anything, but I can definitely tell you there is nothing to stress about. The interviews are very laid back and only a half hour each. The one interview is just a file/extracurricular review, and the second one is like the "why optometry?" portion. My interviewer asked me only three questions before he asked what I wanted to know about IU! There are no additional essays. You really don't need a ton of preparation; if you can explain why you want to be an optometrist you will do just fine.
I am usually a worrier, and I couldn't sleep the night before because I had stressed myself out so much... when I had finished the two interviews I realized how absolutely unnecessary all that worrying had been. 🙂
Best of luck with your interview! PM me if you have more questions.