With Meg's stance on poetry, Castiel should have expected that singing would be considered similarly for her. She would not take well to him singing to her, much like she was not a fan of the poetry he'd tried to recite when she was looking after him in the hospital. It had still been worth an ask, given what she said about why the people in the film were singing. He assumed the same could be said for real life, when it came to people not having the right words to explain things they felt. It was a subject that didn't last long, thanks to the power going out and he was certain they would have to search out means to light the room, so they weren't left in the dark until the power decided to come back on. He knew that these things were temperamental, after all.
He obeyed her order, like the dutiful solider he could be at times. He was good at taking orders and, when he'd been captain of the garrison, giving them. He'd been curious about what she was doing, because she was moving about the various rooms of her home, until coming back to the central location of the bedroom. When each of the candles ended up in their position and were lit, he realized she'd sought them out on her own, rather than ask him for help. He would have gladly assisted her in finding the candles or flashlights to light the room, but she knew her home better than he did. In the time he'd been staying with her during his cold, he'd spent most of his time in her bed and typically in slumber, thanks to the horrible tasting medicine she'd given him.
Castiel watched as the warm glow filled the room and the way it danced off her skin, reminding him of another time when they were lighted similarly. Their first meeting. When they were enemies, on opposite sides of a war and now, things were vastly different. He didn't know the things that were running through her mind, but as she got closer, he didn't pull away. Instead, his gaze fell on her lips, something he didn't withdraw from but embraced. That soft kiss, he reciprocated it, albeit quickly, because she spoke right after.
"I cannot take credit for the power going out. It was simply an act of nature."