We want to make a couple of magical spheres hover around this woman's head:
First, we'll add our magical spheres to the scene. With the above image set as the background (we used wfmm background for improved output over the standard Image background shader), the result is this:
Which is not particularly convincing. The red sphere doesn't appear to cast a shadow (because there's nothing to receive said shadow), and the green sphere is supposed to be behind her hair. wfmm catcher to the rescue! What we'll do is add geometry that roughly effects the shape of the woman. As this is just a simple example, we only need a VERY rough shape, so we'll use a stretched sphere. Here's a grab of the scene in solid mode, and a render:
This seems to be enough; the white sphere is catching a shadow, and blocking the green sphere. It's even casting a shadow on the green sphere, this will help the final result look more convincing. In order to get that final result though, we need to apply the wfmm catcher reflectance shader to the white sphere:
And that's all there is to it. If we had video of the woman moving around, we would animate our stand-in head object too, effectively tracking the video.