Photography is the painting of light.  Just like a painter use paint, photographers paint with light.  They use light to shape light patterns on their subjects face to create depth and interest.  It is the comparison of highlights and shadows. 

The key to outdoor light is to find it and create with it.  The best time of day to take images is at sunrise and sunset because the sun at that time is not as harsh lessening the risk of harsh light and harsh shadows on your subject's face and leading to a less than desirable image.

When I seek out a location the first thing I do is block light.  I block it from the top with a building, trees, or an overhang; what ever you can see and I block one side of the face/body.  This will create the shadow/light that you are looking for to create the dimension on your subjects.  I also use an off camera flash on my subjects  to make them pop in the photograph; to balance the sunlight with the flash.  I have learned this skill by using a light meter(an important photographer's tool) to measure the amount of light on my subject and to measure the percent of light falling on my subject to help illuminate the subject.

In this image, I used only natural light, but followed the rules I talk about above; blocked the overhead and one side of my subject which created a nice Rembrandt light pattern on the right side of his face.