Create realistic contrast
For realism, your characters need to look like they’ve been illuminated by the same light source or sources as the camera in the main pic. Go to the Toolbox and click on the Burn Tool to make it active. This tool can be used to darken pixels depending on their tonality by setting the Range between Highlights, Midtones and Shadows. For this pic we kept the Range to either Midtones or Shadows and set the Exposure to 3% for a really gradual effect. Then, with a soft brush, you can paint over the areas you want to darken to create the illusion that it’s in shade. You can also use the Dodge Tool in a similar fashion, setting the Range to either Highlights or Midtones to paint in highlights on your model.