Congrats on your interview, I will be starting at Midwestern in a few weeks. My interview last fall was 2 professors and I. They make you feel comfortable and let you know they are there to talk to you and find out who you are. They have access to your essay and your application but I don't think they see your grades.
The interview is first thing in the morning so be sure you wake up early and get a healthy breakfast. (There is a Starbucks down the street FYI.)
They will ask questions such as why optometry, why Midwestern, how do you feel you fit in and what you will bring to your class. They ask about the differences between Optometrists, ophthalmologists and opticians and how each fit into the care of the patient.
Since they have access to your application and essay, be prepared to talk about them. They asked me about my experiences with optometry and some specific things from my essay.
They also let you explain anything that might be negative in your application such as low grades or anything but it's optional.
Last they give you a chance to ask questions so be sure to have some!
The rest of the day is going to be a tour of the campus and the clinic and presentations of the optometry program.
I really liked the small campus feel and how everyone was very friendly. The optometry class is pretty small so everyone knows everyone. I was impressed by the labs we have access to use and the clinic was awesome.
Good luck! If you have any other questions let me know!