Over on The (Comic) Beat, screenwriter Todd Alcott shared his very detailed and interesting insight on The Avengers in his post
Everything you EVER wanted to know about THE AVENGERS movie
The Oscar nominations were announced the other day. To no one’s surprise, the screenplay for The Avengers was not among them. That’s a shame, because the screenplay for The Avengers is a startling model of precision, density and propulsion. It manages to juggle no fewer than ten wildly disparate main characters in its ensemble cast and give each of them weight, clarity and purpose. Dear readers, I’ve worked on many a comic-book movie, none of which ever got near production. To get one superhero narrative to work is damn near impossible; The Avengers soars with seven.
It's amazing how much thought goes into every little scene, action or dialogue. That's why screenwriting is so difficult: you have to show, insinuate, create unconscious impressions in the viewer.