CIS gauges how effectively you can be understandable by the standardized patient and also speak grammatically correct and eloquent enough to convince the patient to take a decision or to counsel him for his bad habits. It also builds the rapport between the patient and the doctor and builds confidentiality. The mannerisms of the doctor and professionalism also builds trust with the patient (for example looking into the eye of the patient while talking).
Most of the CIS components get judged in the last few minutes of the 15 minute encounter during closure the case. If the student has wasted time before the 15 min final call, he may not have time to close the case. Several unfinished cases can result in CIS failure.