Imagine your OTP is "just friends." However, one night Person A gets very drunk when out with Person B and other friends. The other friends laugh at Person A's stumbling and don't seem to care much, but Person B's obvious concern and care for Person A reveal that they like Person A as much more than a friend.
"Ugh, why do I go drinking with you lightweights?" Santana mused, leaning on Brittany as they exited the bar. Adam merely raised an eyebrow at her as he kept a secure arm around Kurt, who was listing to one side. Rachel was giggling, seated on Artie's lap due to the fact that she wasn't quite steady on her feet anymore. Sam hadn't had much to drink himself, but he was busy trying to wrangle Blaine, who was the happiest, bounciest, sloppiest drunk Santana knew.
With a smile, Brittany kissed her cheek. "Because you love us," she said and Santana couldn't help but smile.
"I love you," she told Brittany and Rachel cooed at them as she leaned back on Artie, who did not seem to mind at all.
"I love everyone!" Blaine announced with a laugh, spinning around and bouncing off a wall, stubling and falling to the pavement with a thump. His laughter cut off as he said, "Ow!"
He sounded so surprised that everyone giggled, everyone but Sam, who fell to his knees beside him and said, "Dude, you okay?"
"No!" Blaine said, sounding distressed and young, hissing as Sam ran a hand over his leg. "I scraped my knee."
Sam nodded seriously. "I see that," he agreed, rubbing Blaine's shoulder gently. "Nothing's broken though! That's good."
"It hurts," Blaine murmured, slumping against Sam, who was more than willing to hug him.
Santana exchanged a glance with Adam, both of them sober enough to be intrigued, especially when Sam gathered Blaine up in his arms, apparently deciding to carry him the rest of the way to the train station. Blaine didn't object to this, winding his arms around Sam's neck and burying his face against his throat as Sam made soothing little hushing noises and, yep, kissed the side of his head.
Just friends and roommates, yeah right. They might not have acted on anything, but Santana knew love when she saw it.