Or perhaps existing objects in the new scene may affect the way a shadow interacts with its new surroundings. When this happens, it is typical for an object to look out of place in its environment. Retouching can "fix" reality.
In a typical production environment, you might encounter a variety of these types of retouching scenarios: taking an image that was shot in a studio and placing it into a natural outdoor scene; dropping an image into a scene with a different background altogether, which may have been lit differently; cropping an image from an outdoor scene and bringing it indoors. In each case, the color and look of the shadow may have to be adjusted to reflect the changes in the color or feel of the image being brought in and its new surroundings.
Creating realistic shadows from scratch can present a real challenge. First of all, you have to create something that doesn't exist yet, or at least doesn't exist in the form you want it to. The shadow has to be brushed in with typically no point of reference and is totally dependent on the imagination of the person retouching the image. (If you have a limited imagination, this can be a problem, but we'll discuss means of getting around that in this tutorial.)
I will typically start my shadow off by first determining where my shadow is going to fall based on the theoretical light source and the way that light would have been striking the object. The source of light will determine where the shadow will be placed, based on the time of day and the mood of the shot. We'll also want to determine the length and softness of the shadow.