Character Analysis
Actor Jeremy Renner famously wasn't hugely happy with his comparatively small role in The Avengers…and we can't say as we blame him.
He gets zombified in the first scene, spends Acts I and II as Loki's eager flunky, and only gets a chance to earn some payback once the aliens actually arrive. He's terse, has a slightly ironic sense of humor and is really, really good with a bow and arrow. Beyond that, there's just his desire to get even with Loki.
The important thing is that "getting even" doesn't get in the way of the more important job of saving the Earth. Like Black Widow, Hawkeye started out as a villain in Marvel Comics before seeing the light and coming over to the good guys. Here, that means setting aside his very real grievances for the sake of the big picture. Witness how quickly he falls in line behind Captain America when the invasion starts:
CAPTAIN AMERICA: You think you can hold them off?
HAWKEYE: Captain, it would be my genuine pleasure.
It's a small comment but it's a strong one, suggesting that he's a guy who knows how much is depending on him, and he needs to make sure it's all done the right way.