On the cold, snowy days of winter, or those hot, sticky days of summer, or anytime in between, daylight coming through a window can provide a great light source for informal portraits. Some of its advantages include availability, unaffected by weather, a natural and familiar setting for the subject, soft lighting, and the window itself can act as an effective prop. Window light also provides a very flattering light source for people of all ages, especially the elderly because it tends to soften features and smooth out textured skin.
Two things to remember: You want the subject's face to be illuminated by the window, and unless you want the reflection of the window as a prop, the camera should not be aimed toward the window.
Speaking of the elderly, window light is an appropriate light source when taking pictures of residents in a nursing home. No elaborate lighting setup is required, and pleasing environmental portraits can be taken without the person having to leave home.
For best window light portraits, use a window away from the direct rays of the sun, or take the pictures at a time or on a day in which there is no direct sunlight coming through a selected window. Sunlight coming in a window is harsh and contrasty, and will result in reduced shadow detail, strong highlights, and, in many cases, strong texture in the person's skin. In other words, all the adverse effects of bright sunlight outdoors.
Also, when selecting a window, choose one that is as large as possible. The larger the window, the softer the lighting, and the more options you'll have with regard to placing the subject. A small window will act like a spotlight, resulting in stronger shadows and more directional lighting.
If there is no other way of avoiding strong sunlight coming through a window, large or small, it might be possible to eliminate the direct rays of the sun or tone them down with the use of blinds or curtains. But use either of these methods as a last resort because they will reduce the intensity of light, making it more difficult for the camera to select a shutter speed that will adequately reduce the effects of camera movement. Also, reduced light will probably cause the flash to be activated. Flash would fill in the shadows, thus reducing the effect of why you chose to use window light in the first place. So shut off the flash, or don't activate it at all.
This brings up another point. In window light portraits, you want the window to be the only light source. All lights in the room should be turned off for two reasons. First, tungsten or fluorescent light mixed with daylight could possibly cause some weird and interesting, but not flattering, coloration effects on the skin. Secondly, illuminated objects in the background can be distracting to the overall portrait effect. And thirdly (okay, three reasons), with the subject near the window, he or she will be more prominent against the darker background.
Incidentally, you'll want to check the background for objectionable objects or reflecting surfaces that could be distracting. Move as much stuff as possible out of the way, unless the stuff adds interest to the concept of the picture.
Another potential problem is eyeglasses. You are bound to get reflections in glasses, but because of the nature of this type of photograph, these reflections are usually not too objectionable. You will avoid reflections altogether if you have one side of the person's face toward you in profile.
If you can choose a corner room with a large window or set of windows on one wall and an equal sized or smaller window on an adjacent wall, you can use the second window as a fill-in light source. With your back to the secondary window, you will get softer shadows, and the additional light will also allow you to use a faster shutter speed.