Good luck to everyone applying RD. I just completed my RD interview (applied late September, interview notification on 8 December), where questions were similar to what the ED's were asked: Why Columbia, why now, leadership examples, curriculum preferences, personal conflict example, ethical conflict example, alternative (re: back-up) career plans, career goals, etc. As was the case with most applicants, my interviewer was very professional, open and honest. He left me with a good impression, both of the Columbia program and my own chances for admission (though he rightly noted that, with so many competitive applicants, admission is always far from certain). The most valuable things I did to prepare for my interview were to: 1) Speak with current students & alumni about the program; 2) Really understand Columbia's brand and curricular offerings; 3) Know my own story (provide a narrative to your resume and MBA decision); 4) Have good questions for your interviewer.
I hope this helps, and again, good luck to all my fellow RDer's out there.