I got the idea for this fanfiction listening to a song of the same title, 'Mercy Me' by Alkaline Trio. It's based loosely on a few of the lyrics which I have added in here, more for my own train of thought than yours but if you want to check out the song then feel free. I like exploring Castiel's emotions as a wayward angel, so I hope you like it, please review/rate and let me know what you think :) thanks very much for reading!

Mercy Me

I've lost faith in everything, I'm lost, so lost, I'm lost without you

He blinked, the feeling of eyelashes brushing against his cheeks an unusual sensation. He swallowed, his throat dry and scratchy. His chest felt tight, his muscles felt sore. This was the first time he had ever felt his emotions in such a physical manner. For him, the two had never collided, never been in tandem, because before he had been so spiritual before.

But now his powers were gone, he was human, and everything was so physical and raw. The heat and the cold were enough to drive him insane, the feeling of bristly hair on his jaw and the headaches from a lack of sleep. The papercut on his thumb made him twinge in pain whenever he picked anything up. The feeling of emptiness and hunger made him feel weak and pathetic. Thirst was something completely unknown to him.

And the alcohol had never affected him as much as it had before. He was what Dean described as a 'lightweight', though once he had fallen over the first time he then referred to him as a 'deadweight', which had confused the former angel no end.

But perhaps what was worse than anything was the feeling of fear. Castiel had often felt uncertainty and on the rare occasion, sometimes even apprehension. But now he had lost his powers, and now that every step they took drew them closer to Lucifer, Castiel began to feel an awful darkness inside of him, making his breathing short, his muscles twitching, and he felt as though a hole had appeared inside of his stomach like a vacuum.

The physical feeling of fear was so much worse than the emotion. So raw, so frightening. Castiel was scared.

I've never had a problem with facing fear, but I'm done, over and out, my dear

The first time Castiel's fear was unveiled to the Winchesters was when Castiel had fallen asleep in the back of the car, as he had been doing a lot now he was reduced to human, having to run on sleep and food, instead of being an angel and running on his blessed battery power.

Castiel was dozing, his head resting against the back of the seat as his eyelids drooped down slowly and he began to softly snore. He recalled distantly hearing Sam say something about his snoring, and Dean mutter something about angels, and a moment later there was darkness.

Castiel had never had a nightmare before. He didn't know that it was a nightmare until he had woken up. It all seemed so real.

The first thing he saw in his dream was Lucifer and Michael, in the bodies of Sam and Dean, fighting tooth and nail. Castiel was watching a little way away, and he was trying his hardest to stand up and help Dean, or to yell out to them, but his movements were so slow and sluggish, and when he spoke no words came out of his mouth. He was unable to shout, unable to scream.

And then there was a blinding flash of white light, and there stood Sam, while his brother lay on the floor with a dagger to the heart, and blood began to slowly seep into the clothing of his shirt. Castiel could see that Michael was gone and it was Dean, alone and afraid and bleeding to death as his heart furiously pumped around the sharp knife in his chest.

Oh mercy me, God bless catastrophe

And Castiel woke himself with an awful, choking cry as he shot up in the back seat of the car, making Sam jump awake and Dean swerve the car slightly. "Holy crap, Cas!" he exclaimed, as he righted the wheel, and glanced at his friend in the rear vision mirror. "Are you alright?"

It took Castiel a very long moment to regain a slight amount of composure, as he realised that what he had seen was not real, and that he was still riding in the back of the Impala, that Dean was still driving, that they were on their way to Detroit.

Castiel swallowed, his throat feeling tighter and dryer than before, and he croaked, "Yes. I'm alright."

The concerned frown and worried glaze to Dean's green eyes said that he knew it wasn't alright, but Castiel had looked away, too ashamed to admit that he wasn't alright. Sam had raised an eyebrow but didn't say anything.

There's no way in hell we'll ever live to see through this

It was a few hours later when they arrived in Detroit. Sam had immediately begun planning for his ambush of Lucifer, while Bobby helped him out. Castiel waited in the car, feeling very tired but refusing to go to sleep because he was so scared he would see that horrible scene again.

He nearly hit the roof when there was a sharp knock on the window of the opposite door, and Castiel spun to see Dean staring in the window. Dean frowned at Castiel's unusually wide eyes, and he opened the door and jumped in the back seat. "Hey, Cas," he said, concerned, "What's wrong? You've been shaky since we got here."

"You didn't tell me about nightmares," Castiel said instantly, knowing that he couldn't pretend he was okay for much longer.

Dean blinked, a little surprised at Castiel's accusation. "You knew about them," he said. "I told you, I had nightmares after I got back from hell."

Castiel stared at his hands in his lap, and he said in an alarmingly small voice, "Yes, but you didn't tell me how it felt."

An expression of realisation passed over Dean's features. "Ah," he said. "You've discovered one of the many wonders of being a human."

Castiel glanced at his friend. "Wonders?"

"I'm being sarcastic, Cas," Dean said. "It's not a wonder. Nightmares are god damn awful. I'm sorry."

"I – I didn't realise how much pain you were going through," Castiel said, frowning as he understood how Dean felt for so long after being raised from perdition. In retrospect, the nightmare wasn't all that scary, but at the time Castiel had felt a cold fear in his veins he never thought possible. "I'm sorry I wasn't more sympathetic towards you, Dean."

Dean shrugged a little awkwardly. "Nah, don't beat yourself up about it Cas," he said. "That was then. This is now. Are you okay?"

"No, Dean," Castiel replied. "I'm scared, and I'm afraid, and I'm frightened. I've never felt these things before. It's so – weak."

Dean shook his head. "No, Cas. It shows you care. And for that I'm grateful."

Castiel stared at his strange friend for a moment, trying to figure out how Dean could compose himself so well, how he could fake his emotions so well all the time. It was such an art. It was hard enough being able to control the emotions that are there without having to pretend that they don't exist at all. Dean seemed to be able to transform his fear into energy, into fuel for fighting.

And Dean got out of the car, and Castiel realised that if there was ever going to be a time to fuel up for a fight, now was it. Time to let all the anger and fear and sadness bubble and boil, because soon he'd need it. He'd need it to help his friends, to help the world. Because even a world with nightmares is a world worth saving.

So drive yourself insane tonight, it's not that far away
And I just filled up your tank earlier today