Context: Takes place during Season 4, before Sam and Dean really get to know Cas. They're still getting to know each other. Prompt for the angel feather came from Episode 8x12, where a feather was an ingredient for a spell.

"Hey, Dean," Sam called from the hotel table, where he was looking at a thick, battered book. "Take a look at this."

Dean ambled over from the TV, holding a half-empty beer in one hand and a half eaten burger in the other.

"Whatcha got?" He asked, taking a swig of beer.

Sam grimaced at his brother's choice of nutrition but turned back to the thick open book in front of him. "I've been reading through this book Bobby loaned me. At first I thought it was just a book on philosophy and the interactions of humans, demons, and angels. Actually, it does point out an interesting correlation between-"

"Sam. Point." Dean slid into the chair next to Sam.

"Right. Okay, so here in this chapter I found a small section on spells. And they're not simple spells, either. Most of this is pretty intense stuff and requires objects like," Sam ran a finger down a column of evenly-spaced, old-fashioned script, reading carefully, "a fossilized speckled goat's jaw, a scorched painite stone, and some sort of flower that I'm pretty sure you could only get in 1596 on the only blue moon of the decade. The point is, this stuff is hard to get, and most of these spells are, well, virtually impossible. But take a look at this one; here."

Sam scooted his chair over as Dean leaned towards him and looked at the words Sam was pointing to.

"Facie ad abscondere a malo." Dean read aloud. "Well what the hell does that mean?"

"It's Latin."

"Yeah, I got that."

Sam rolled his eyes. "It's a disappearing spell, Dean. It's a spell that will block the spell-caster from the 'sight of evil eyes'" Sam translated. "And from what I can tell, it should work. For a while, at least. And we've got most of the ingredients already."

Dean straightened. "Well that could be helpful. Wait, most?" he said, pushing the burger away, suddenly focused.

"Yeah." said Sam. "Check out the last ingredient."

Dean slid the book closer and looked where Sam pointed. Next to the last bit of text was a tiny drawing of…

"A feather?" he said questioningly.

"Yeah. But not just any feather. In angelo est aequalitas pluma." said Sam, reading out loud.

"An angel's feather."

Dean shook his head and stared at Sam. "No way. Are you telling me this means that angels…have feathers?"

Sam shrugged. "It makes sense. I mean, we know angels have wings, right? So why not feathers? Look at this." He pulled over his laptop and opened a file so that several pictures of angels popped onto the screen.

"Almost every picture we have of angels depicts them having feathered wings. I'm not saying every angel does, but these are from the lore. And how often has that been wrong, Dean?"

Dean leaned back and let out a deep rush of breath. "Wow. Okay, so, does the lore say anything about getting one of these feathers? Is there a ritual or something?"

"Not that I can see," said Sam, dragging the book back and scanning the page again. "But, we have all the rest of the ingredients in the trunk. And we do happen to know an angel."

"So, what, we pray to Cas, ask him to pop down here and just, what, hand over a feather?" Dean gestured.

Sam shrugged. "Why not? I mean, information about angels is already pretty hard to find and it's not like their feathers are just lying around."

"Okay," Dean said after a moment's pause, "Let's do this."

"You want me to what?"

Apparently, just straight up asking an angel for a feather was not a typical request. Cas stood in front of the boys, looking both surprised and confused.

Sam broke in. "We need the feather for this hiding spell, and we've got all of the ingredients, except the last one, which…is a feather from an angel." He finished cautiously.

"A feather from an angel's wing is a powerful object, Sam. If used incorrectly, it could cause devastation on a worldwide scale." Cas said seriously.

Dean, who initially asked Cas for the feather, spoke up. "OK, look, Cas. We're not trying to cause offense or anything. And, I mean, we've got a pretty good idea about how powerful these kinds of things can be."

Cas looked at Dean.

"Look," Dean looked at Sam and back to Cas. "The spell, it's a protection spell. And me and Sam, we think it could keep the bad guys off our track for a little while, just to help us out while we figure out what to do about, you know, the apocalypse."

There was silence in the room. Cas looked contemplative but said nothing.

Sam said, "Yeah, we just need one. For the spell. That's all we want to use it for."

Sam and Dean both looked at Cas.

"Alright." he finally said. "But only one." He closed his eyes and Sam and Dean both took a small step back out of habit.

But Cas simply held out his hand for a moment. Then, he uncurled his fingers.

In his palm was a long, pristine, white, sharply tipped feather. Sam and Dean stared at it for a moment.

"Here." Cas said simply, and offered the feather to Dean.

Dean gingerly lifted the feather out of his hand. "Thanks, Cas." he said. "You're welcome. But be careful." Cas said.

And then, with a whoosh, he was gone.

Sam and Dean looked at each other for a moment, then at the feather. "Well, I guess it's true!" Sam geeked out a little bit.

"Dude, I don't believe this. The guy has actual feathers. Who knew?" Dean said, twirling the feather between his fingers.

"Yeah. Hey, let me see it."said Sam. Dean gave him the feather and suddenly started to chuckle.

"What?" Sam said, looking up from studying the feather under the dim hotel light.

"Nothing, it's just, the angels- no wonder they're so uptight. Must be hard, having all those feathers brushing against your backside all the time." Sam made a please stop talking face and Dean chuckled as he turned back to his abandoned burger.