I've said that soft light is a great thing for portraiture, but there are times when you want to create a dramatic mood with darkness and pools of light sculpting your model. Before we talk about a specific set-up, let me explain the gear that makes this very easy to to:
|Alien Bee 800 strobe|
Strobes I think are vital. Strobists love the portability and quick set-up using flash units, but a flash does not put out nearly as much light as a studio strobe can. The added power is important so you can stop the lens way down to f/11 or f/16 and effectively darken a normally-lit room. I use Alien Bee strobes, and even their lowest-power 400ws strobe is strong enough to achieve this effect. If you have a B800 or B1600, you can create a darkened environment outdoors in daylight! The more lights you apply to your subject, the less shadow you'll have and those shadows will be filled in, or minimized by the added angles.
Grids help you place light where you want it, and keep it dark where you don't want light. The standard reflector on my strobe sends a beam of light out at 70 degrees, with a great deal of light scattering at an even wider angle. Simply blocking that beam with barn-door attachments or flags can block direct light, but they lack the precision to place a tight beam of light where you want it. Grids are honeycomb-shaped openings in front of the strobe and limit the direction of that light. They come in different sizes to create smaller and smaller spots of light, and are described by the angle of the cone of light. So, if a photographer asks you to 'hand me that 20-grid', he is referring to the grid that puts out a 20-degree cone of light. I have a set of grids that I use on every photo session: 40-grid, 20-grid and 10-grid.
Here is a very simple setup, with one strobe pointed at Rossi on a boom directly overhead using a 20-grid. One strobe in a fully-lit room lets you create this kind of drama: