See this competently done and nicely edited trailer above? Made completely within the confines of GTA V - a world so flexible and huge that you can make movie trailers, nature documentaries, and pretty much anything else you can imagine.

It's a little unclear whether or not this is actually a complete movie being made by someone, but at the very least, it's a pretty nice trailer - and a solid argument for what a ridiculously great job Rockstar did with bringing Los Santos to life.