Wow!  Rarely am I as floored as I was when I popped in Half-Life for the first time!  This is done with the Quake 2 engine!  I certainly couldn't tell!  When you first get off the train in the beginning, a security guard  TALKS to you, and his mouth even moves (although one of the scientists  looks like he talks out of the SIDE of his mouth)!  Of course, other games  do this now, but Half-Life was one of the first when it came out, and MY  mouth dropped open!  When I entered the facility, another guard talked and  told me that his computer crashed and was going to get it fixed!  Trust me,  there's PLENTY of other scenes like this that will make you wonder if it's  a movie or a game.  Everything is just so REALISTIC!  What's the catch?  ...NOTHING!  Listen to the gaming magazines and web sites...buy it!  You'll  be glad you did!