I'm dubious about how much of an impact the interview has your chances. While it's true that schools consistently it rank it as one of the most important factors, adcom members on these forums have said that in reality just about everyone does well on the interview and that it's impossible to separate people into groups just based off of interview performance. It makes logical sense too, considering the questions are (usually) easy and most people are capable of performing well in an interview. Furthermore, most med school interviews are extremely short; only 20-60 minutes per interview, and you usually don't have very many of them. In MSTP admissions which often have upwards of six interviews spread over 2-3 days, the adcom members on the MSTP forum have still said that the interview is mostly useless for making acceptance decisions. Again, that's logical. You can't get to know a person in the space of 20-60 minutes, especially in a setting where everyone is on guard and trying to impress you.
IMO the interview is only ranked most important by schools because if you bomb it there's no way you're getting accepted. However, doing well on it does virtually nothing to help your chances. As long as you perform acceptably on the interview, the school just goes right back to making its decisions based on pre-interview information. So basically the interview can only hurt you.