Should your answers to interview questions match what you wrote in your PS perfectly? Is there room for some additional stuff that was not mentioned in the PS?
That assumes premeds are normal human beings. Just take a breath and focus on answering the question asked with your mind being cluttered with what you said in the PS, or the secondary, or what you think an adcom wants to hear, or some vague thought on saying something that you have an irrational idea may sway the interviewer. Just put brain in gear before engaging mouth. If you get a question you dont know or haven't thought about previously, then say I haven't thought about it previously and think it thru with something like "well on first thought..." Don't try to figure out where the interviewer is going, whether he/she is giving you an opposing view because that is what they think or setting you up in a confrontational question. Just answer it. If they give you a different view that you think really seems better, then say so. If you think something else then say so.I don't think the point of the interview is to be so formulaic. Just speak like a normal human being and you'll be fine.