The circumstances under which.
notes: The first sentence has been bugging be absolutely forever, and I finally managed to do something about it, yay! And I was also so tired of all my fics being old and incredibly sucky, so. Comments &c will be loved forever.
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The circumstances under which Crowley and Aziraphale became Crowley-and-Aziraphale were never quite clear to any concerned, possibly least of all to Crowley and Aziraphale themselves.
It wasn't some sudden, grand change in the way they bantered and drank or how they looked at each other. There was no gazing at each other and sighing lovelornly, no dramatic outbursts or confessions from either party, as far as anyone knew. The change was gradual, like spring slipping into summer before one even realised. The change in the way Crowley and Aziraphale acted and spoke and were wasn't anything awfully startling or scandalising, very possibly to Crowley's great annoyance once he figured it out, most like around the time everyone else did, too.
The change was small things, like the way Crowley very nearly quit trying to annoy the angel and his clientele by making the books in the angel's collection rather more risqué than they originally were or the way the angel almost stopped trying to guilt Crowley out of various deeds (not that it had worked too well to begin with), or, perhaps most noticeably, the way they started standing just inside each other's personal space, apparently rather unconsciously and naturally.