Taking pictures of people in stock photos can be very tricky. There are many various factors that come to play in order to make a photo look sells worthy. Stock photos of people need to have lighting that looks perfect and is perfectly lit. Harsh shadows are a definite no in stock photos unless you are specifically trying to get that specified look.
If you take a look at stock photos of people they all usually have diffused lighting. The transition from light to dark is seamless and very soft. The lighting is usually very even throughout the entire picture. Shadows are never present unless desired. Stock photography will require some good knowledge of lighting properly.
A good start for lighting people would be the key light. This light will be the main light of illumination of your person of interest. Placing the key light on the same axis of the lens will make less shadows and a more flat picture. This type of light placing will make for very even lighting and can reduce blemishes and such. Another option is to place the key light to the side of the subject thus creating side lighting. Side lighting can be very harsh and very diffused. It all depends on the fill light.
The fill lights primary purpose is to fill in the shadows that are cast from the key light. An actual fill light can be used or reflectors can be used as well. The fill light is placed usually on the other side of the subject. It is on the opposite side of where the key light is at. The fill light must have a light intensity that is either equal to or less than the key light. If it is more than the key light it becomes the key light.
A back light can also be used to create a halo effect around the subjects outline. This will distinguish the person from their background. A background light also can be used to light the background. You can have many lights for a single photograph. It all depends on the desired effect you want.
When lighting for people in stock photography or any type of photography you can always default to the high key lighting with even balance of light and such. However, is this really going to help show the subject better? More advanced lighting actually is for the purpose of showing the character of the person being photographed. You can use lighting to make a person look scary, powerful, weak, happy, etc. Lighting can make a person look like they have more of a personality. Using lighting to add impact to your stock photos will be very beneficial and can make your photo much more high quality and profitable.
The color of light used is very important as well. Using the right color of light can make a subject look healthier and more vibrant. Using the wrong light hue will result in a sickly looking person. Do not simply skip out on the details of light when taking stock photos.