This is pretty spoilery for Age of Ultron, so if you haven't seen it yet, come back after you do. You have been warned!
This is sort of an attempt to reconcile how... um, abrupt Bruce's exit from the film was. More to the point, I'm kind of inclined to think that maybe it wasn't Bruce, so much as it was the Hulk, who decided they needed to go hang out on a deserted island for a while.
Usual disclaimers apply.
He never talked much about what his transformations were like. Most people were too uncomfortable to ask, and mostly he didn't like to think about it. It was painful, of course, physically; like his muscles were on fire and his bones were stretching and twisting, and his entire body was trying to pull itself apart… but that wasn't the worst of it. Not by a long shot.
The worst was what happened inside, in his mind, because when the Other Guy took over, Bruce Banner was nothing more than a passenger, even at the best of times; and later, he would be forced to admit that this qualified. The Other Guy just sat there, his massive frame curled forward along the shape of the Quinjet, watching the world rush by in a blur below them. For the first time in his tumultuous existence, the Incredible Hulk had actively chosen contemplative silence.
Locked away inside him, Bruce Banner was the one screaming. The irony, as it turned out, was not lost on either of them.
Bruce had always hated this part the most, this sheer helplessness; he didn't know where to begin describing what it was like. He felt everything the Hulk felt. Fists and bullets and lasers and collapsing concrete, showers of glass pricking into his skin… it was horrible. He always felt like he should be dead, but then he would wake up back in the real world, without a scratch on him. Everyone thought it was the transformation that took all the wind out of him and left him so utterly exhausted afterwards, but it wasn't. It was the ghost of the Hulk's pain that lasted the longest and hurt the worst. But he would happily take all of that, if only he could be the one choosing where to run, or who to punch.
Perhaps it was just his mind trying to keep him sane though this impossible madness, but he always found himself in a small, dark room. Surrounded by padded walls on three sides, there was no door, and the only windows were the kind that they used on the upper floors of skyscrapers; inch-thick glass welded to sturdy metal frames that could not be opened. The Hulk could have smashed through them with one fist as an after-thought, but he was never the Other Guy here. Here, he was always just Bruce Banner, and no matter how hard he pounded on the glass with his fists, and how many times he screamed himself hoarse, he could never break through. Eventually, he would retreat to a corner in defeat, squeezing his eyes shut to try and keep from seeing what was happening on the other side of the glass. It didn't matter. The windows weren't real. The room wasn't really there. There was nowhere to run, and no matter how tightly he closed his eyes, the highlight reel played on. He didn't usually remember most of it, he reflected, though often when someone explained what had happened, he would fill a specific detail before he was told. This was his proof that he didn't just black out when it happened.
He dreamed about that padded room sometimes, and for a long time, he had lived in mortal fear that he was transforming in his sleep without realizing it. That fear had eventually abated, but the Other Guy never felt closer to the surface than when he was dreaming.
Sometimes he wondered where the Other Guy went when he was in control. Did he have a padded room with unbreakable windows too? That thought made him uncomfortable. It was the closest he got to feeling sympathy for the Hulk.
The padded room was always cold, and he always felt naked, even if he wasn't; he felt that way now, huddled in the corner, shaking uncontrollably. He had never been locked away so long. He didn't sense the passage of time here, not in the normal sense, but he knew it had been longer than he'd ever gone before. It was beginning to scare him. He couldn't find the thin thread of control to tug that would bring him back to himself, no matter how hard he tried. He tried meditating. He tried to calm himself down. But the Other Guy's strange calm had defeated his own.
He had wondered before if there was some point of no return; if the moment the Hulk became calm and aware enough, capable of shutting him away as Bruce had always done to him, he might someday transform and never change back. He wouldn't blame the Other Guy for the impulse. It's what he would have done, given the chance; no, it was what he had done. So who was the real monster, then? Because Bruce was the one sitting there wondering if that moment had arrived, and nothing had ever scared him more than the idea of being trapped in this padded room for the rest of the Other Guy's life.
"Safe."
The Hulk's grumble cut through the surge of panic swirling through his mind. He was so startled, he responded, "What?"
"Banner safe."
That took him aback. The Other Guy hardly ever talked; he seemed to find it easier to communicate in grunts and growls and expressions, in ape-like body language. And he had never tried to talk to Bruce before.
"I don't understand."
The Other Guy made a few frustrated grunting noises. The frustration wasn't directed at him, Bruce realized, but at the Hulk himself, at his inability to speak. Finally, he seemed to find the words he was looking for.
"Hulk keep Banner safe…"
"I was safe. I was safe with my friends, I was safe with her," Bruce argued automatically. He'd quite forgotten who he was talking to. He no longer cared. He just wanted the Other Guy to take them back.
The Other Guy made another grunt that sounded like dissent, "Hulk keep Banner safe… from Banner."
"I… I still don't…"
"Banner hate Banner… when Hulk goes away… afraid to hurt… N-"
He garbled her name, but the fact that he had tried to say it at all was… well, the Other Guy had never been so verbose. And he was just beginning to realize how aware he was of what was going on when Bruce was in the pilot's seat.
"You want to take us away, so we won't hurt Natasha?"
"And Shiny Man," the Hulk added quickly, "And… others. All others. So Banner… stop hating… Banner."
He'd never seriously entertained Tony's notions about the Hulk until that moment. He supposed he'd have to thank his friend one day. Because Tony was right. The Hulk was his bodyguard and his defender, and he was willing to do whatever it took to protect Bruce. Even from himself.
"We'll never stop hurting everyone, will we?" Bruce's voice sounded broken and hollow to his own ears. The Other Guy gave a noncommittal grunt.
"Not… Banner fault."
"Yes it is."
There was nothing but silence for a long time. Finally, Bruce spoke again, "I thought you didn't like me."
His answering grunt had a note of amusement to it, "Puny Banner."
"Yeah, yeah," Bruce couldn't help but chuckle a little.
"Puny Banner," the Other Guy repeated, as the Quinjet flew on in silence, "Hulk keep safe."
Thoughts are nice, if you like. Especially all those nice ones I've gotten from y'all for previous stories, thanks a ton for that :)