Compositing in Motion
Here's the deal: I film something, in this case a person talking, and leave the camera in a fixed position. Since I filmed the person in front of a green screen I can key out the green and place my own background instead.
Inside Final Cut: Motion, I can layer the talking person video over the background in 3D space. Now, you can't change the perspective of the keyed out video, but you don't have to! A trick I've learned after years of simulating 3D in Flash animation is that you can pan from side to side, up and down, even front to back without giving away the fact that you're still in 2D. In real life there is a certain amount of perspective shift that would happen if you pan from one side of a cube to the other. This is important to note! Agressive panning from side to side will break the illusion, but if you keep in mind the orignal point of view of the camera it will work like this.
I filmed this person off center to the right, so I keep my "digital" camera's movement to right of center. I don't pan any further to the left, because you would expect to see SOME amount of the back of his head.
The most import concept here though, is: Don't rotate the camera! If you angle the camera at all, up, down, left or right, your video will look like a cardboard cutout. (In the video I did this to show you the depth between layers and to illustrate this concept.)
Watch this video to get a better idea of what is happening, and while you're at it, notice the "parralax shift" as the the cammera dollys. The effect is pretty convincing.