Rather than do any interview prep course, I would honestly just sit down and have a real hard think about why you want to do medicine as well as what you have planned out for yourself in the future. Think about your past, when did you start wanting to become a doctor and what led you to make that decision, think about what you have done in order to show that you'll one day become a good doctor. Then think about the future, what do you hope to one day achieve.
Think about the skills required in a doctor and think about whether you have those skills, think about what those skills entail and how you've developed them over time. Think about your interests, hobbies, what you like doing...etc.
Don't go into an interview and lie, if you have to be dishonest or rehearsed in order to pass it, then you're better off not going into medicine, because it looks like your desires and passions lie elsewhere, be honest, be yourself and say what you believe is right, not what you think will get you into medical school.