Why the Hulk works in The Avengers

The hints of Bruce Banner's struggle to accept that he is the Hulk make his character compelling but his focus on the team—rather than himself—makes him watchable. Bruce doesn't talk about himself  much in the movie, but when he does, he sets up an interesting contrast in his identities as Bruce and the Hulk.

Spoilers after the cut.

He refers to the Hulk as "the other guy" in two significant scenes. The first is when Natasha asks him to help S.H.I.E.L.D.

Bruce says, "What if I said no?"

Natasha responds with, "I'll persuade you."

"And what if the... other guy says no?"

In this scene, Bruce uses the Hulk as a bargaining piece, as his out. Natasha might be able to force Bruce to go with her, but she doesn't have any power over the Hulk. Bruce marks a distinction between himself as a person and the Hulk as a powerful entity.

The other scene is on the airship with all of the Avengers gathered in the lab. 

Bruce says, "I got low! I didn't see an end, so I put a bullet in my mouth... and the other guy spit it out!" In this case, the Hulk is something that will not leave him alone, something out of his control.

Bruce knows that he and the Hulk are the same being, but he makes a distinction between the human and the creature. Why?

One possible reason is, he hates that he turns into the Hulk, so he denies that the Hulk is a part of him. Part of that denial is drawing a line between what's Bruce and what's the Hulk. Bruce, however, can control when he turns into the Hulk (for the most part), so that makes me think he doesn't hate the creature.

If not denial, then maybe Bruce makes the distinction for other people. He wants others to understand that Bruce is one personality and the Hulk is another. Calling the Hulk "the other guy," then, is telling them to be careful, to know that he doesn't simply go through a physical change.

It's psychological too. All of the other Avengers stay consistent in their identities, whether or not they're in superhero mode. Bruce is the only one that changes. He said as much:

"I don't get a suit of armor. I'm exposed...like a nerve. It's a nightmare."

These brief scenes hint at the inner workings of Bruce Banner without taking away from the larger story.