Please excuse the subject in the above photo, you shoot what's available and that's usually me. I wanted to illustrate a one light portrait and it's progression. In this one light portrait, I progressively subtracted light within the scene to create a more dramatic image. Let me start out by showing the setup for the final and 4th photo shown above.
I used a FourSquare™ for the light source of these portraits. The stool in the room is about 6 feet in front of the background. The first photo of the 4 shown above was shot just using the FourSquare™. Light traveled to the background and bounced off the shooting space wall on camera right.
In the second photo, I added a black panel behind the FourSquare™ in order to block some of the light off the background. In the setup photo above you can see the shadow being created by the panel behind the FourSquare™
In the third photo, I added a black cloth to kill the bounce off of the wall on the right. And in the final image, I added a flag to block light off of my right shoulder and across my lower neck.
Next time your using a single light for a portrait, try flagging some of the light off of your subject in order to create a more dramatic portrait.