People often comment on how a painting has captured the essence, the soul of a person. Most of the time this is
utter nonsense. Because painted portraits don't have captions, people fill in the meaning themselves. And so they read all sorts of deep insights into the squint of the eyes, the elusive smile, the slight frown, the position of the little finger, etc, etc, etc. These insights are in the mind of the beholder, not in the painting. Different viewers see different traits in the same portrait. Nothing wrong with that.
The artist can help to stimulate the viewer by making sure that each portrait contains some of the following three features:
1. it resembles the person depicted (more about that in the future)
2. it contains an idea, refers to something, creates an intriguing atmosphere
3. it is sufficiently pleasing to the eye to revisit the portrait a couple of times
These features can compensate for one another to a certain degree, but good portraits have at least some of each.