Quote: "Could you give any example of how these are to be used?"
First, you probraly need an screen recorder. (Fraps, CamStudio, whatever)
Then you need a green screen program (Sony Vegas 9.0, Windows Movie Maker (with addons), Adobe After Effects...etc)
Then record walking animations, reload, shoot ...etc
Then make a REAL LIFE VIDEO or record a video of you standing still ...like walking....and ETC
Then combine, and you will have an Real Life FPS Creator game...
That was only an idea.
Gutsplatt Games
EDIT:
Quote: "The only thing I can think of would be any sort of machinima. I mean, simply keying that out would make it really easy to apply video to the green spaces."
Right before i posted :S