For sakond and those who feel they interview strongly--do you mind sharing thoughts on what make your interviews strong?
How do you balance advocating for yourself with being humble?
How do you affirm that the interview is going well/poorly?
My current supervisor told me that I am a very strong interviewer (he chose me over 20+ applicants for this job) and I am currently 1 for 1 in terms of acceptances/post-interview decisions rendered, so at the very least, I'm a decent interviewer.
My personal feeling is that while you are there to sell yourself, you are not a salesman and the interview is not a pitch session. I think the most important thing you can do is be prepared for the interview. That means knowing your AMCAS and secondary applications front and back. Also, rehearse, but don't script, your answers to questions you are decently sure are coming (why medicine, why X institution, etc.) Make sure you do at least some basic research about the school (history, major accomplishments, etc.) from their website just in case they ask you school-specific questions. Finally, KNOW YOUR RED FLAGS. They are likely to come up and you want to be able to explain why they shouldn't affect your candidacy. By doing this preparation, you are less nervous during the interview and sound more confident in your delivery of your responses to the questions.
In terms of selling yourself, make it sound like what you've done so far is important but not revolutionary (which is true for most people). Try and frame your responses in terms of "I" or "we" and use more active verbs in describing activities (managed, supervised, recruited, conducted as opposed to participated, attended, observed, etc.). Don't be overly humble because if you think what you was unimportant, chances are the adcom will too.
I typically gauge the strength of the interview by observing the interviewer. If they look bored or uninterested, obviously those are not great signs. If it's part of your disposition, try to be funny (appropriately so of course) to lighten the mood and make the interview more enjoyable. That's all I can think of right now, but feel free to PM me if you have more questions.