Yes, the term is compositing and even though I'm not a VFX guy I think it would go something like this:
1. Shoot the background w/no one in it.
2. Shoot the actors that are going to freeze on a green screen.
3. Shoot the actors that are going to keep walking on a green screen.
4. Go into an application like After Effects, Shake, Flame, etc., and combine all the layers together so it looks like a single shot.
Lethal