Not to downplay any of this, but this seems like standard compositing done in most movies, and especially TV shows with smaller budgets. Big set pieces, or shooting on location is expensive. The more you can fake, the cheaper it is. For example, any shot in a stadium, or a place like Times Square is often much simpler to fake than to go out there and film on location.<p>World War Z breakdown:
<a href="http://www.youtube.com/watch?v=NTr0k6-8_Fw" rel="nofollow">http://www.youtube.com/watch?v=NTr0k6-8_Fw</a>