just had my alumni interview (submitted 1/2 invited to interview 1/14, but took this long to schedule!)
word of advice: i think the columbia interview instructions tell you to wait a week for reply -- do not wait a week for reply. two business days.
i was asked many questions that were posted on this forum already -- ethical dilemma, leadership style, teamwork example, example of a time i faced adversity. feel free to PM me for even more specific questions
how i prepared: i prepared five or six stories that i could fit or frame into leadership / teamwork questions, if that makes sense. i basically went through my college and work experience and thought of the most meaningful events that occurred in that period and evaluated my role in them. and then i saw how i could weave them in, according to question type (leadership / teamwork / difficult time with coworker / etc.)
i definitely mangled the 'why mba' question even though i should have most easily prepared for that one! i was nervous at the beginning though but thankfully my interviewer was very patient and encouraging so that i relaxed as the interview went on

BEST OF LUCK to you all!