Movie plots or scenes usually get leaked to the public well before the film comes out in the theater, but as for the Dark Knight, there was no way that was going to happen. Even Batman himself wouldn't have been able to convince the producers to take home a script.
Maggie Gyllenhaal says when she was approached with the role of attorney Rachel Dawes in the movie, the movie was driven to her house by an assistant. Then the assistant had to sit in her driveway until she finished it. Only problem was, Maggie was a new mom tending her little girl Ramona, so it took her 20 hours to finish! And the guy stayed in her driveway the entire time. That's dedication! (And a little bit anal). But it's definitely worked, since the public only knows what we've seen in the trailer.