I think the biggest mistake many people make is that they do not answer the question. It is really very simple....just answer the question, nothing more, nothing less. If you answer specifically what you are asked and that is not sufficient, the interviewer can question you further for more information. I find this to be an irritating habit of many people and not just during interviews. I often ask questions that require simply a yes or a no, and instead I get a 5-10 minute discourse that somewhat rambles around the question that was asked.