Picking up the best candidate in an interview is always a challenge for recruiters. Numerous theories, experiments, observations and experiences have been written around the same.
I think, one thing particularly useful in an interview is to judge a candidate from his stories. Ask questions that open him up. One that makes him speak his life in front of you. His life stories (of course, related to the attributes you are searching for) can be the best ‘casual’ way (and less susceptible to gamification by the candidate) to judge a candidate.
His logic, emotions, attitude, skills etc. can be judged through well crafted sequential questions. The usual questions like ‘tell me one instance where you have showed strong leadership qualities’ are helpful but not that much.
The interviewer needs to weave a path based on what he wants to test the candidate on. He himself has to be convinced of his ‘story of questions‘. Then engaging the candidate in a conversation through these questions is again an art which comes through careful planning and practice.