I always feel that people are one of the most intriguing and difficult subject matter to capture, particularly making sure that you not only capture the likeness of the subject, but the emotion as well.  There are many subtleties that can change an expression such as the portrayal of the eyes, the positioning of the mouth in a smile or at rest, the use of body language, or simply the use of shadow and light to convey the solidness of the human figure.  These are things that make capturing the human form challenging, but also rewarding in the process of painting which is very satisfying upon completion of the work.