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.
7 Types of Lighting in Portrait Photography
How to Use a Key Light for Extraordinary Portrait Photos
Brian Smith's Portrait Photography Advice in PhotoShelter Guide
Dynamic High-Key And Low-Key Portrait Lighting Tutorial
Key Lights, Fill Lights, Hair Lights and More Photography Strobes
Three-Point Portraiture Lighting and Beyond
13 Tips for AMAZING Dark Photography (+ PRO Ideas)
Free Online Photo Course: Lindsay Adler's One Light Artistry
Studio Lighting For Beginners - The Three Light Setup
10 creative self-portrait techniques that will inspire you to create your own
How to Shoot Key Light Photography (Photography Lighting 101)