Originally Posted by Black Ice
and if you die you start the game over? I only rented it but that bugged me.
To answer Bowser's question, you don't HAVE to do anything. If you wanted, you could hang out on the helipad for the full 72 hours, although that would be boring and stupid. What you do throughout the game (missions, saving people, etc.) just determines which ending you get when the 72 hour mark hits.
As to Black Ice's question, if you die, you can either start from your last save, or start the game completely over. The one advantage to starting over is that you keep your stats, so if you die as a level 10, you start the game over at level 10, so the games a bit easier at the start. Also, if you finish the game, you can start a new game at the same level you ended the previous game. I've played the game through twice now, and during the second playthrough, I was almost unstoppable.