A prime example, and perhaps one of the most infamous, is when Loki shouts to Hulk:
"I am a god you dull creature and I will not be bullied by--"
Before getting his ass SMASHED! LOL. This would not have been half as epic if it weren't for the dialogue, the painful smashing and the em dash that jarred the viewer into the unexpected. And that's it, really: people like little surprises and that's what em dashes do--they incur the unexpected. When done right, of course.
Avengers is a prime example of the em dash done right for comedic effect. Concurrently, it is the only movie where the Hulk doesn't suck. And that's also a beautiful thing.