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There is only one known possible way an angel is capable of possessing a human vessel: The human has to allow the angel to inhabit the body with verbal confirmation.
Heaven chose the master strategist— angel of Thursday— Castiel, to reach into the depths of hell and rescue Dean's soul. He was a strong warrior, being graced the opportunity to witness much of Earth's historical events. He ran his own battalion of angels for a period of time, he followed heavens orders without question or doubt, and angels held high regards for him.
When Dean Winchester's screams his throat hoarse in Hell for help, in technicality, it was a yes.
Getting into hell was the easiest part of his journey. Heat battered his wings as he narrowly avoided demons, sensing the angelic presence all the way from the deepest depths. The angels that accompanied and protected Castiel struggled to keep them at bay.
“Stick to the plan, the mission.” Castiel ordered. Minds now focused, they began searching through muddled prayers in the vast expanse of tortured souls for the one heaven needed.
Castiel and his army fought their way through demons with spilled blood from both sides. Time moved different in both heaven and earth, spending mere hours in their pursuit. They had to move faster. The longer the soldiers spent in Hell, the more demons would learn they were there and pounce on them. Castiel pushed forward relentlessly, smiting anything in his way with determination.
Finally, Castiel approached what could be interpreted similar to a prison cell. In hell, the metaphysical aspects of the room wouldn't make a whole lot of sense to a mortal. While there was no iron bars, there was a void of suffering that surrounded the captive inside. The aura felt like it could consume anything that touched it.
Somebody help me!
Even through the personal hell, an impossibly bright light of green and gold batted away the darkness around Dean's vessel, fighting it off. The light inside spun and reverberated against his limbs. The strength, anger, and pure hope of this human’s soul was enough to temporarily keep it from being twisted into a demon. For the first time in Castiel's eons of existence, he was baffled at the sight.
Reminding himself of the task at hand, the Seraph turned to check behind him. A few of his warriors were stationed to protect him, blades in hand and energy pouring from their hands. They were willing to buy him enough time, at the risk of their lives. Castiel faced the cell once more, taking a deep breath when he knew he didn't have to.
His mind pressed through the gates of darkness, honing in on Dean's soul infront of him. It was like a fog in the void at first. The choking he sensed was surely amplified versions trauma from the man's memories sought to destroy the angel like it was a virus. The angel's essence tustled against the horrors and reached out to Dean.
Castiel managed to grasp Dean's shoulder. The soul inside immediately felt this intrusion, ceasing it's attack against it's surroundings to target the hand placed on him. The pain was indescribable, for both the angel and the human. The grace from Castiel melded with Dean, a burning sensation searing his palm into the skin, and ringing of the angels pure form mashed against the soul in retaliation.
Castiel had to get this under control before this soul booted him. Channeling as much focus as he could, he blindly felt around his humanity. He searched some sense of security, some kind of emotion that would temporarily sooth the man so he could pull him from the depths of fear.
Human emotion was always the one thing Castiel never understood, that he couldn't understand. He felt lost as he witnessed memories pound him like flash grenades.
The first image the angel saw was Dean's mother, holding a younger version of him in his arms with a smile. The warmth that radiates from her felt like it could've been the best hug in the world. He then saw his father figure sat near a lake, fishing rod in hand. A sense of maturity and the need to impress flooded Castiel like the water the fish swam in. He then watched a younger brother, sleeves left unbuttoned, carving his signature inside a black Impala. Much like heaven, this car gave him a sense of Home. The last memory he witnessed was the same boy, all grown up and pulling a blonde woman's waist with mirth.
Dean's soul softened, it too being able to see these memories. It's energy buzzed against Castiel's hold with confliction. These people Castiel caught glimpse of brought it the solstice it needed. The angel urged further into his mind, no longer trying to fight against the soul. Instead, his grace provided company and melded deeper into Dean's subconscious.
At last, with one last surge of Castiel's grace, he found himself seeing through the Winchester's eyes in the darkness. The hands of the vessel tugged rough against the chains that suspended Dean, breaking them from the binds and swam through the black void towards the exit.
Who the hell are you?
Castiel heard the voice of the man echo in their mind, but didn't respond. The irony in his words was tempting to entertain, but he needed to keep as much energy as the could to get Dean out of here.
The possessed human reached the front of the cell's hypothetical door, and saw the vessel of Jimmy being guarded by another angel from their battalion. The two vessels made eye contact for a moment, before Castiel tried to thrust their hand through that thick wall for an out.
At first, it didn't work. Castiel clawed with hands that ultimately weren't the angels. The could feel the panic of the man inside shaking their limbs, so finally, they sent a message to speak to Dean.
You have to work with me. I can't do this if you're unwilling to be saved.
A moment passed of conflict that danced on the expression of Dean's face, the human soul unsure whether to truly trust this. After all it's seen while trapped in hell, the soul had a hard time trusting there wasn't a catch to this. Their hand pressed against the walls that kept them in, Castiel allowing Dean to ground themselves against it. The emotions Dean held twisted against the angel, coiling around everything they knew.
Okay.
That was the only recognition the angel received before the soul inside them surged against their rugged fingertips. Castiel urged the grace into their shared palm to aid in the force applied to the barrier. The electricity of blue and green cracked this wall with, and finally it crumbled under the pressure.
Dean's vessel stumbled into hell's hallway, falling down to his knees breathless. The angel immediately tried ripped himself away from the soul, causing an overwhelming feeling of loss to trap the grace. Dean and the man beside him both winced. Trying again, Castiel slowly eased away from the soul, and the pain eased. A flash of blue appeared in Castiel's eyes, and he was back in his own vessel.
“Castiel, we don't have much longer.” The soldier released the angel now that he was back in the right body, before glancing at the man they saved.
Dean was still recovering from the experience, adrenaline coursing through his veins as he reached up to hold his shoulder with utter confusion. Castiel stood upright, following the gaze before his blue eyes met green. He could still see that soul inside of him, glistening against the iris of his green eyes.
Offering a hand to the mortal, Castiel helped Dean onto his feet before taking a more defensive posture. “Unfortunately, that wasn't yet the end of our mission.” The angel spoke, turning towards the angels that fought down farther in the hall. His eyes glowed with rage, raising a hand to smite them.
Instinctively, Dean tucked his face into the nook in his arms to hide from the light, ears ringing when trying to catch the words between the angels. He was forced to keep his face buried into arm, feeling himself be grabbed from behind. A palm was held over the shoulder Castiel had gripped earlier. He could feel the raw skin scab over with a healing tingle. At the sensation, he felt as though he was flying.
Dean wouldn't know he literally was, the angel clinging onto him with a sense of protection. The other angels either finished their fights or retreated to join Castiel in their escape from Hell. They soared through flames and ash, faster than he could register.
And then, Dean felt himself take a breath of air for the first time, from his grave.