MARISSA

So he immediately meets up with Aziraphale and makes a plan to stop it and as soon as they plan to work together, Crowley starts to feel optimistic:

Then they spend the next eleven years trying to stop the apocalypse and for a little way they may start to think that it's all going to be okay but then they realize.

So then it's time to switch gears. Things are becoming more dire and he's starting to lose hope so when he and Aziraphale head to Tadfield, he's already switching into his most cantankerous version of himself, knowing that every moment he wasn't uniquely focused on preventing the apocalypse could be catastrophic. He tries to build up an emotional wall between him and Aziraphale and he fails almost right away. At the first sign of Aziraphale's classically trained puppy dog eyes, his resolve cracks.

After he's reminded of Aziraphale's loyalties to heaven, i.e. 'weight to a moral argument,' he tries to double down by giving the humans real guns, yet he still can't bring himself to lie when Aziraphale asks him if they're killing each other. And when Aziraphale points out Crowley's privately good heart, the demon thinks, "This is my chance. Gotta really sell it." So, he does this little number.

He bares his teeth and snarls about how very not nice he is while uncharacteristically behaving in what would typically be seen in a violent manner.

And it doesn't work. Aziraphale sees him growling and hissing and all he's aware of is their contact. He still looks at Crowley like he's kind... He's still looking at him like he might just smile if Crowley ever had the nerve to tell him that he means, not just the world, but also the universe to him.

He tries throughout the rest of the episode to maintain his facade of angst and indifference but after this, it's half-hearted at best. There's a subtle nod to it all right before Anathema hits the Bentley.

It's been suggested that demons are incapable of detecting love as a palpable emotion but it's the specific wording that denotes something deeper.

Why wouldn't it feel out of the ordinary if you were surrounded by love suddenly? Well, maybe if that's something that you experienced all the time in certain company.

Then comes the biggest revelation of them all! His hot and cold attitude, the shoving, the vehement denial of his kindness towards both Aziraphale and humans. It's all for a reason. This is the one folks.

His love, in his eyes, is dangerous to the cause. There's too much at stake and he's so emotionally volatile at the idea of never being able to, uninterrupted, be with this creature he loves that he's terrified to give into it for even a moment, lest that moment be just what Heaven and Hell need to take it all away from him for good. To take Aziraphale from him.