This is a myth promoted by universities to prevent you from preparing, because if you do prepare, their job becomes harder.
If you believe this myth, try watching a stand-up comedy show by the same comedian for two days in a row. You will see that the seemingly 'random' thoughts which pop into their heads, which makes their lines so funny, are exactly the same. They have been rehearsed and practised hundreds and perhaps thousands of times. No one would say that stand-up comedians come across as rehearsed! Jerry Seinfeld, for example, takes one line of delivery and works on it for days, polishing the intonation, delivery, pitch etc. Yet when he comes on stage, it seems natural.
MedEntry's Medical Interview Training Sessions are designed to help you prepare effectively for your interview and come across in a natural, genuine manner.