You may take the exact same shot multiple times, but varying the angle and degree of lighting on the subject can create a drastically different mood for each shot.
The Portrait Photographer's Quick-ish Guide to Studio Lighting Ratios
Dynamic High-Key And Low-Key Portrait Lighting Tutorial
The Portrait: Positioning of the Key Light
Key Lights, Fill Lights, Hair Lights and More Photography Strobes
How to Use a Key Light for Extraordinary Portrait Photos
How to Shoot Key Light Photography (Photography Lighting 101)
The Portrait Photographer: Key Light and Fill Light for Portraits
How to Use a Key Light for Extraordinary Portrait Photos
13 Tips for AMAZING Dark Photography (+ PRO Ideas)
How to Use Fill Light in Photography
How to Use Fill Light in Photography