When describing it in writing, subtle is best. Readers will "get" the tiny things that make the characters realize they're getting hooked on each other. For an example, I used a conversation between two brothers in our October book to show that the hero's brother realized something was up. Again, it was subtle--more an admission of his own life, but the hero got the point. See if this helps:
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It wasn’t until he and David were walking to the truck to go home that his brother commented. There was no particular tone attached to the question---he kept his voice carefully neutral. "You’re worried about her, aren’t you?"
Other than his mother, his brother was the one most likely to spot a lie. But an acknowledgment wouldn’t hurt. Adam looked up at the starred sky as he opened the door. They weren’t as bright as down there. It was something he’d noticed when he was hunting the deer. He shrugged. "She’s tough. I’m sure she’s fine."
"It’s a damned slippery slope."
He didn’t comment further, so Adam turned to him after they were both in the cab and buckled up. "What is?"
David started the truck and turned on the headlights. "It was during that three-alarm fire last fall for me." Adam looked at his brother questioningly as they turned out of the parking lot. He’d just opened his mouth to ask, when David continued. "I knew Bonnie was on shift that night, and when I started to hear calls for ambulances—" He paused and stepped harder on the accelerator to bring them up to speed on the road. "I know a bunch of guys at the firehouse . . . hell, I have friends in three firehouses, don’tcha know. But I was only listening for one name on the box when they reported the roof collapsed." He turned his head after they were stopped at the red light and had a small smile on his face. "Her hair still smelled like smoke when I asked her out on a real date the next day. Like I say . . . slippery slope, bro."
Adam couldn’t think of any response, other than, "Fer sure."
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Falling in love usually happens in increments. You remember to call the person, but forget to call your mother. You find yourself buying the brand of soda to stock in the fridge that you know they like. You growl a little or feel twitchy when someone flirts with them where you can see/hear. It helps to have a third party observe the changes, like, "Why are you buying Pepsi, Bob? You hate Pepsi." Etc., etc.
Does that help any?