Aziraphale knows Crowley better than anyone else.
And right he should. He's had thousands of years to get to know him, to memorize his mannerisms, his likes and dislikes, to learn to predict the way he'll react to things. Crowley has done the same for him. It's written in the little ways that Crowley alters his routine for Aziraphale, in the small selfless things he does to make him more comfortable. It's in the way he'll turn up on a Sunday afternoon with a paper bag of pastries from Aziraphale's favorite bakery across town, and the way he'll sit sprawled out on the bookshop sofa with his cup of black coffee, waiting patiently while Aziraphale eats them, savoring every bite.
It's funny how, even after all these years, he's still learning new things about Crowley. Like how, for a being who claims not to read, he has an astonishing number of books in his flat, most pertaining to astronomy. Or that when he's having trouble sleeping he'll slither up to the ceiling and curl up there, hanging over his bed like some kind of infernal bat. Or that once in the early 1990s he tried to tempt Fred Rogers and found himself so outgunned by the man's inherent kindness that Crowley ended up being the one questioning his purpose by the end of it.
Those realizations don't matter, though. The one that absolutely tears Aziraphale to pieces is that Crowley is terrified of being alone.
It takes him longer than he wants to admit to see it, and once he does he kicks himself for not realizing sooner. It explains so much of what Crowley has done over the years, the little things that Aziraphale has always thought his reactions were extremely out of proportion with.
It's in the lines of Crowley's face when circumstance forces them apart, no matter how briefly. It's in the way Crowley never really lets him out of his sight, changing which time-faded piece of furniture he's lazing about on in the bookshop so they're always occupying the same space.
It's in the tiny, needful way he whimpers in his sleep when Aziraphale rolls over in the night, reaching out for him blindly. Aziraphale softens in that moment, taking Crowley's hand and pulling him close, running his fingers through his dark red hair, something Crowley would never let him do if he were conscious for fear of messing it up.
"It's alright, dear," Aziraphale murmurs in the darkness, holding onto him tightly, and Crowley curls in against his chest. "I'm here. I'm not going anywhere."