Windows provide a beautiful light for portraits and by choosing the side of the house that you’re shooting on, you can actually vary the quality and color of the light that you’re using. North-facing windows, for example, produce a gentle even lighting, though the color tends to be on the cool side much of the year. East and west windows provide gentle warm light early and late in the day (respectively) and for much of the day any windows facing south will provide a relatively strong and warm light. In this photo my cat is facing west and watching the sunset and so her face is filled with a very pretty warm light (and she really does watch the sunset most days which is one of the reasons I love cats so much--they get the mystery of it all!).
With the exception of light from north-facing windows, however, light coming in through a window can get quite bold and contrasty. One way to handle this contrast issue is to have your subject face the window (the cat chose that position on her own) and then shoot them either from profile or, if you have room to maneuver, by getting slightly in front of your subject. By doing this you’re turning the window light into a front light and this illuminates the important parts of the face. Alternatively you can sit your subject sideways (so the light is just illumination one side of their face) and then use a sheet of white art board to bounce light back into the shadow side of the face. Just aim the reflector so that it bounces light from the window back toward the dark side of your subject. You can use flash too, of course, but you have to be careful not to let it overpower the window light or you'll lose the drama of having the window be the primary lighting source.
Photo Notes: This photo was shot with a Nikon D90 and a 70-300mm Nikkor zoom and was handheld. Exposure was 1/80 second at f/5.6 and I used a small amount of fill flash with -2 stops of compensation on the fill. My latest book is Exposure Photo Workshop, 2nd edition and it has lots of good info about lighting, lighting direction and using flash.
This is posting #501.