Yeh, I was an instructor at ATP..
The interview is to weed out eh.. the OBVIOUS people who won't ever make it in the aviation field.. I.E. guys with 52 piercings in their nose, etc..
We also give the quick critical engine thing to see if "comprehension ablilities" to see if you can be shown something and grasp it fairly quickly and regurgitate what you've been shown..
The system they have sorta sucks, I remember we all hated giving interviews.. We were not paid for it, we usually had flying to get done.. it was just something extra we had to do and had very little time for it.. Basically, for the interview you get whatever instructors "day it was.." we each took a day - to be "office manager." Any fun stuff - interviews, written tests, bank deposits, etc were our duties for our day..
One thing many have said in this thread is extremely true - often times the quality of the student was directly related to the quality of the instructor.. I never did the 90 day career guys.. I only worked with add-on ratings and ATP ratings..
I wore the shirt probably 75/25.. It was tucked probably 75/25.. I don't think that detracted from my ability to instruct though..

The fact you dressed up is good, we had some kids who would show up in shorts and a tee shirt..