Remember when they face the Devil?
He smiled at Crowley.
"I’d just like to say," he said, "if we don’t get out of this, that … I’ll have known, deep down inside, that there was a spark of goodness in you.”
"That’s right," said Crowley bitterly. "Make my day."
Aziraphale held out his hand.
"Nice knowing you," he said.
Crowley took it.
"Here’s to the next time," he said. "And … Aziraphale?"
"Just remember I’ll have known that, deep down inside, you were just enough of a bastard to be worth liking.”
The moment when you realise they’re already talking about each other in the past tense.