So like…imagine Aziraphale for the first time seeing someone throw salt over their left shoulder to ward off demons, because they believe that evil spirits lurk on the left. And Aziraphale realizing that Crowley always sits to his left, and isn’t that interesting? Crowley doesn’t have a problem with salt in general, but what is this “warding off” business? Does it make demons sneeze? Or run away? Or does it just annoy them in some way? Of course, if he asks Crowley, he’s not going to get a straight answer, he knows that. So Aziraphale tells himself not to think about it, which works about as well as it always does.
Crowley quickly realizes something’s up because after thousands of years together, he knows that when he sees Aziraphale frowning in a certain way that Aziraphale is trying to talk himself out of doing something ridiculous. So Crowley makes up his mind that whatever Aziraphale does, he’s going to use it to fluster the angel, because that’s arguably Crowley’s main hobby.
So eventually, Aziraphale of course gives in to temptation and yeets some salt over his left shoulder at Crowley, and Crowley has prepared his inner drama queen for whatever this was going to be, so he gives a yell and fakes some sort of allergic reaction. And of course, Aziraphale freaks out and Crowley congratulates himself for having flustered the angel.
But the thing is, Aziraphale also knows all the faces that Crowley makes, and he knows he’s faking it. But Aziraphale is all too happy to play along, because what Crowley’s actually done is to give Aziraphale some ammunition. Now whenever Aziraphale “accidentally” spills salt on Crowley, Crowley has to either go through the whole dramatic act again or admit that he was faking it, and Aziraphale is quite happy with either of those outcomes, because he’s learned that yes, throwing salt over your left shoulder does in fact annoy one certain demon, which, coincidentally, is Aziraphale’s main hobby.