A smile does make a difference during an interview, but only when it is done earnestly.
Some candidates are so worried about answering the questions correctly, that they often forget to smile. It makes a difference when you smile, as you are projecting yourself as an accessible and affable person to your potential employer. It is also a sign of confidence and shows that you possess enthusiasm for the job you want. But make sure that your smile is not an artificial grin: that will merely give the impression that you are not genuine!