The Nice And Accurate Stories Of The Ineffable Husbands: Good Omens

But in case they do...

So exactly, Aziraphale didn't fall in love with Crowley on 1941, it was way before, he simply realized that love was what he felt for the demon that year.

It's 1862, that day on St. James Park where Crowley asked for his 'insurance', holy water. Aziraphale was not going to bring the only thing that could kill his best friend to him, no way. It's not that he didn't trust Crowley, of course he did, but this time his heart, if any angels make use of it, was telling him not to. "What if it burns him to death? I would be sad if he died, he's my best friend, why would I give him something that could kill him so easily?, No, no, no, I can't do that to Crowley, even if he gets mad, I can't lose him" even after denying completely to Crowley the holy water, Aziraphale's head was a mess, for what would a demon want holy water? What did he exactly mean with insurance? If they never checked, why would they now?
This, however ended in an argument between the two of them, Crowley felt so betrayed, why wouldn't the angel give it to him? He was just trying to protect himself, just in case, you know, maybe they check one day. Crowley was furious now, he could only trust his best friend for this, he needed his help, when it had usually been otherwise, and now the help he needed was being denied, with the extra of losing Aziraphale because he thought it was more of a suicide pill rather than protection. Also, the fact that for the first time Aziraphale brought up the topic of "I would be in trouble if they find out I have been fraternising" hurt the demon's feelings. 5000 and something years of friendship and suddenly now they were fraternising. What a stupid way to lose the one you love.
Aziraphale was really not worried about heaven finding out, it was just an excuse, but it didn't go as he intended, in fact, he had just lost Crowley, he could feel he had offended him, but if that's what it took to keep him safe, that's what he would get. In fact this made Aziraphale feel different, he had broken friendships before, but he was specially worried about this one, he actually liked Crowley, we could say, that's when he started to realize, he might have been in love with his best friend for some time now, but he was more worried about the present, he was worried about Crowley.

Then 1941 came by, when Aziraphale, who seem to have everything planned, was surprised by the double agent, on a church, with books in between. He felt quite embarrassed, until somehow, Crowley appeared. A demon, in holy territory, stupid. But there he was, and he came to save... Him? Why? They hadn't seen each other in almost 100 years, was he still mad at him for not giving him the holy water? Or maybe he wasn't there to save him at all, after all it was a church, it had the holy water the demon was after. Anyway Crowley, Anthony J Crowley, saved him that day and that exact day, when the demon was good enough to remember saving his books, that day was when Aziraphale was hit with the big 'I actually love him' like if it was a brick wall, and he reconsidered the holy water thing once again, he didn't want Crowley to get hurt, and he now knew he'd probably do anything for it, so maybe he should make his job easier.
But back to Aziraphale's feelings. Angels are beings of love, or so they say, so it was actually normal if Aziraphale loved Crowley, but what was quite new to him is that it was not only love what he had for the demon, he felt romantic atraction, like some humans towards each other, he wanted to be with him, he actually needed him in his life, and he would do almost anything for him. He, like in some books he read, now daydreamt about spending the days and nights with him, romantically, as partners, not best friends. But again, the thing they always said, an angel and a demon, if Heaven or Hell found out... Not even God would know what they'd do to them. But Aziraphale now had reconsidered everything. If giving Crowley the holy water was gonna get him back, he would give it to him, but he'd wait a little, he didn't wanna make it too obvious, he'd wait for the right time.

That time was 1967 in Soho, London.
When Aziraphale heard that someone named Anthony was planning a robbery at a church, he knew that had to be Crowley, so he decided the moment had come.
He waited at his car, if Crowley wanted holy water, he was gonna get the holiest, taken from Heaven itself in hopes Crowley wouldn't do anything stupid with it and it might at least fix their friendship, maybe dine at the Ritz sometime soon, but when Aziraphale saw the friendship didn't actually need a fix, it had never ended or even been in danger, giving him the holy water was nonsense, but still he did.
"You go to fast for me Crowley" that's all Aziraphale could think after realizing his love could endanger Crowley, he had just gave him something that could vanish the demon from Earth's surface forever, he needed time to think.
"Just in case they check, that's all". Aziraphale said to himself.

"Oh so THAT is what happened" said present Crowley. "I must say, angel, I didn't see that coming" Crowley was still too proud to admit Aziraphale had hurt his feelings back then.
"I'm sorry, I didn't want you to die Crowley, I care about you" Aziraphale was impatient, he knew Crowley too well, so he knew what was coming after this.
"Just kiss me already angel"