Doing a portrait has two basic challenges, one is the likeness. Likeness is the representation of the features of the person, for instance, the shape and size of the nose, the eyes in relation to the face. In order to develop the likeness in your portrait, you need to have a solid observation skills. This ability can be acquired through the exercises of drawing/painting from observation.
The second challenge is the personality of the person. This requires the artist's ability to observe and understand the subject matter (the person), and transfer them through the visual languages of drawing/painting into your artwork. Like words, visual languages of drawing/painting has the power of express emotional feelings, these languages include each and the combination of the following: composition, value contrast, lines, shades, lighting and colors, and etc. There are many variables in each above elements, for examples, lines can be short or long, thin or thick, straight or curve, dark or light, many or a few. This ability usually takes much longer time to developed. It takes more than the observation skills.
To prepare a portrait painting, I usually take time to listen and understand the sitter, and to make myself to familiar with the sitter's features and personalilty.
In this quick portrait drawing demonstration, I tried to show my students a drawing process and techniques to capture the likeness and personality in a very short time.