Usually the panel consists of 3 or more professors (mine had 4) each taking turns to ask you questions from the 4 subjects (Microeconomics, Macroeconomics, Mathematics and Statistics).
The first thing I was asked was my academic background and my favorite topic. They start asking you questions from the subject you say is your strongest, finishing with your relatively "weak" subject. You can not afford to screw up the questions they ask from your strong topic (that you yourself have told them). If that happens, you are pretty much out. You can of course make mistakes in topics you have mentioned as your weak ones. I was sitting like a dumbfeck in 2 of the 12 or 13 questions I was asked. But they were from the topics I wasn't good at, and as a result the panel was fine with it.
The questions aren't that difficult. Actually they will be of the same level of difficulty as the written/computer exam. Very simple things will be asked (at least I was), mostly conceptual.
Personally speaking, I felt IGIDR was the easiest exam (I gave IGIDR, JNU SSS, JNU SIS, DSE and ISI)
Hey just one more question. Do they ask general questions like why this course, why this institute, Why should we select you, questions about gap years if there are an? And if they do, do you think answers to these questions add or subtract to your chances of getting selected?