Why do you want to do medicine?

This is a basic question that can be answered in hours or in seconds. Most medical students don't know why they fully want to do medicine until they reach their clinical years. Naturally, the interviewers will not expect you to have everything figured out; after all you are just an 18 year old trying to get on to the course.

But they will anticipate a well informed opinion about the positives of medicine and how that aligns with your qualities. For example, let's say you like science and helping people. 90% of the applicants will say this. To differentiate yourself, give an anecdotal example that is specific to you:  "I worked as a lab assistant and enjoyed the science practiced. However, I felt that something was lacking for me. Through my volunteering with the homeless and the Red cross, I got to work with vulnerable people whom I could help. I saw health professionals in action, which led me down the path to medicine."

Alternatively, saying that you had a completely different interest prior to medicine, but a key event or experience in the other field deterred you from it. "I was part of the debate team and was considering law. However, having interned in a local law firm, I realized that this was not for me... etc"

In an interview it is important to be able to answer this question within a minute. Try to personalize your answer so that it doesn't seem learned or too similar to the other candidates. Being earnest and giving personal reasons, however trivial they may be, is always better than some grandiose story that is not too believable. 

In personal statements you want every sentence to lead back and answer this question, so that the admissions team has no doubt in their mind that you are fit and excited to study medicine and become a doctor.