Most people hire people they like, rather than necessarily the most competent person. Research shows that most decision-makers make their hiring decisions in the first five minutes of an interview and spend the rest of the interview rationalising their choice.
I can tell you from personal experience, that candidates nowadays are extremely well prepared. They know all the standard answer, and they've practiced intelligent replies to even the toughest standard questions. In short, many candidates are much more skilled and much better prepared for the interview than the person sitting across the desk from them (who, honestly, may not be prepared or skilled at all).
I don't want to sound harsh or dramatic but if you don't approach recruitment with a rigorous structure and reliable methodology it can eventually harm your business. Actually, let me clarify this a little. What you have to be, is careful not to succumb to the well-thought-out dance of a candidate or introduce more errors. Remember, you're dealing with biology here.
Because candidates are becoming increasingly sophisticated, it is not unusual for a company to hire an individual who makes the best impression, rather than the person who is best suited for the position. It’s performance vs. substance.
You see, many interviews unfold as a sales pitch. The candidate is selling himself as the best employee anyone has ever heard of. Once short-term needs are met, such as landing a new job, the candidate is not motivated by them anymore.
You really need an experienced and well prepared HR Manager or third party recruiter, who at the same time is also a careful buyer and a skilled interviewer to cut past the candidate's commercials and BS.