One light – from the front it’s flat, but from the side and back it creates shadow which means you can alter how something looks. You can create form and shape by adding shadow or adding highlights. I like to backlight where I can (on location, the sun does this) but try where possible to add reflected light before a second light, where it’s appropriate. Here’s a few examples from the gallery below, reading from left to right:
Row 1 – Image 1 – single light to the side, soft but no separation; Image 2 harder light with shadows which model the chin; Image 3 – backlight to hair create separation.
Row 2 – Image 1- Backlight is more from above with reflected light (tri-flector) pushing from below and flattening shadows; Image 2- softbox and backlight; Image 3, same but more like sandwich lighting.
Row 3 – Image 1 – single light camera right and high key set means that ration of 2:1 achieved by reflected light alone; Image 2 and 3 – Canon speedlite to the the rear and off camera left.