The second act starts off well for the hero. In Save the Cat, Blake Snyder describes the first half of it as the “promise of the premise.” It’s when the set-up is finished, and the characters have fun, exploring whatever is the hook of the film. It’s when the pretty young people are in love, the new millionaire abuses his newfound wealth, and the alien invasion gets interesting. In the first Harry Potter film, it would be the scenes in which he is exploring Hogwarts and learning how to use magic. This is when Spider-Man becomes a superhero, and has that upside down kiss with Mary Jane.
Snyder suggests that most of the scenes from the trailer happen in this part of the film, which makes sense. This would be before the major spoilers kick in, and after the first act ended the way the audience was primed to think it would end. Things do tend to get worse in the rest of the second act, but revealing those details in the adverts can be problematic. Either it spoils plot twists, or an understanding of the severity of the situation is dependent on a knowledge of the events of the first hour of the film.
In this trailer for Sacha Baron Cohen’s The Dictator, we can guess that the promise of the premise is when the Supreme Leader is transformed into an anonymous New Yorker. The film is slightly unusual in that the set-up in which the dictator abuses his powers in ridiculous ways is pretty funny as well, so the trailer will contain a lot of scenes from the first quarter of the film.
With The Hobbit: An Unexpected Journey, my guess is that the majority of the scenes in the trailer come from early in the second act. It’s when Bilbo has agreed to join Gandalf on the dangerous mission, because a trailer for a fantasy film that’s just about the protagonist’s initial refusal to go on a quest isn’t as interesting as sword-fights or visuals of heroes exploring foreboding ruins.