A lot of study is required before going to the IDC. All the material must be studied beforehand and knowledge reviews completed (13 if I remember correctly) and ready for review. There are several books you need to read i.e. The law and the diving professional, the business of diving, and Children and diving. You will also be required to retake the exams you took for your DM rating, and know the material as the IDC is not persay remedial. You should know the physics, physiology, etc. upside down and backwards. Your skills need to be to a 4 or 5 demonstration quality. You need to know how to tie a bowline, sheet bend and half hitches.
The IE is two intense days of test taking, presentations and water demonstrations. The rescue portion is pass/fail and you are allowed one retake. Mess up one thing in the rescue, say let the victim's head go under water, fail. You have to demonstrate 5 skills chosen by the examiner, you are allowed 1 retake. You are given 2 confined water skills to teach, if you mess up one, you do the other. Same in open water.
No one in my IDC (EASE) was unsuccessful. There were around 3 from other IDC's who had to come back at a later date, and one who was taking an exam retake who failed again and left.
The IDC is fun but again, pretty intense as time is limited. The first 3 or 4 days is the AI portion, the last few, the Instructor prep, learning to teach the PADI way.
Good luck and have fun.