If you've been fortunate enough to have been granted an interview, it goes to show you the school is seriously interested in you. Statistically, you have a 50/50 chance of acceptance at this point.
Your academic ability is no longer the question at hand and instead they want to get to know you as a person and more importantly, your interpersonal skills. The medical school interview is arguably the most important step along the entire process and the most important, yet subjective. In many ways, the interview emulates the patient - doctor relationship. Unless you're going to be a radiologist or pathologist, your interpersonal skills and abilities will be key to your success as a physician. Much like a patient - doctor interaction, during the ~ 20 minute interview, your ability to articulately speak, counsel, educate your audience & mannerisms will define the moment.
You may give the exact same interview to 2 different interviewers and they may view it completely differently. On top of that, how they relay their impression to the committee, there's room for the strength of the message & impression to get lost.
One way I recommend applicants may significantly improve their chances during each interview is to solicit feedback subtely. For example, with the question why do you want to be a doctor - there is no correct answer. Furthermore, you may speak 10-20 minutes without addressing your listener's concerns. That's why I recommend that with questions where there is ambiguity you solicit feedback. You may answer the question briefly for ~ 2 minutes and then ask: have I answered your question or would you like me to expand ? This affords your interview to be more of a dialogue which tends to be more successful. This way, you also know whether or not with much certainty if you've answered your listener's question.