I find it difficult to decide what makes a great portrait. Of course, factors like lighting, location, perspective, and even the reason for taking the photograph lend additional levels of significance to the final image, but the person's story usually makes the whole endeavor more enjoyable. In an ideal situation, I would have already built up a substantial repertoire with the subject to the point where a portrait was feasible, but regrettably that's not always the case. Sometimes you simply have very little time to and you just have to create as you go.