It was a reflex. He couldn't help it. He had kissed the angel.
They had been walking back to the bookshop from the park when it started pouring. Aziraphale had laughed and spun around, practically dancing in the blasted stuff. Crowley had rolled his eyes and continued walking. Aziraphle caught up and took his hand. Crowley very nearly blushed.
When they finally made it to the bookshop, they were drenched. Aziraphale had started to laugh at the disgruntled look on his companions face as they stood dripping on the carpet. Crowley started to get annoyed, but then as he watched the angel, wet with rain and laughing his heart out, he acted on impulse. He grabbed he angel's arm and pulled him into a kiss. It didn't last very long, and it was surprisingly chaste for a demon, but it just about changed the world.
Or their worlds, at least.
In a few seconds Crowley realized what he was doing. He backed off quickly.
"Hoshit. Oh fuck. I-"
He blinked and ran out the door, leaving a startled Aziraphale behind him.
The angel put a hand to his lips and smiled.
Crowley didn't come over to the bookshop for the next few days. In fact, Crowley didn't leave his flat for the next few days. He sat on the couch collecting dust while his mind threatened to implode.
By the end of the week Aziraphale had had enough, and came to the flat. He walked in on Crowley re-re-alphabetizing his already perfectly alphabetized Cd's. Upon seeing the angel he dropped everything he was holding on his foot. Aziraphale bit back a laugh.
"Dear, I think you've avoided me quite long enough."
"Uh, Angel, well, I, um, you see, what I meant was, I-"
Crowley found it rather more difficult to speak with an angels lips pressed against his own.