The six primary emotions of anger, disgust, fear, joy, sadness, and surprise may be a pretty simple way to boil down our emotions. We are much deeper than that and as we grow we learn to control these emotions in various ways. The first way is by controlling the intensity. If you watch a baby you will find an increase in the intensity of emotions for example from concern to anxiety to FEAR to terror. I look at a baby and I see concern in his eyes, as I get closer I can see anxiety as he quickly looks for mom, as I pick him up I begin to see FEAR on his face, and finally when I throw him into the air and catch him you will see genuine terror. (Now, being the observant person I am, I will try to stop at concern by inserting some joy - like candy behind the mom's back). We have intensities in these emotions.
Now look at the six emotions with their mild to extreme intensities and you will find 24 different facial expressions using those 6 as primaries. Look at the picture: Stern to indignation to ANGER to rage. Alert to wonder to SURPRISE and then to shock. All nuances and intensities of the six primaries.
As we get older we gain the nuance of these emotions and attempt to control them but as Dr. Ekman (Lie to me) and Scott McCloud (Making Comics, who drew these emotions) know we cannot really control our emotional facial expressions completely. It will come across as a "strange" look on our face or a insincere smile, etc. What happens is a crossing of these six primaries and when we do, we find even more emotional possibilities based on a combination of FEAR and JOY.