Catch lights add vitality to a portrait.
Catch lights are the reflections of light sources on the surface of the eyes.
If you're using fill flash to brighten shadows, the flash will create catch lights.
If there's no need for fill flash, you can create catch lights by setting the flash exposure compensation from between about -2.0 to -3.0.
You don't want the flash to be bright enough to be visible on the face of the subject.
For more about catch lights, go to Tip: Catch Lights Add Vitality.