You give them subtext.
You give them “Where’s the angel?” “I’m not leaving here without you.” “Where the hell are you man?” “I know you’re in there, We’re family, we need you. I need you.”
You let it simmer.
You give them face-cradling, death of a loved one. You give them “I can’t let you do this.” “Nobody wants him here more than I do.” You give them “He’s in love…” “Don’t lose it over one man.” “You gave up an entire army for one guy.” “It was all about saving one human, right?”
You let it simmer.
Then you pile it on.
You give them “You can’t stand the fact that he’s mine.” “Dean Winchester completes me.” “Our future” “We’ve done extraordinary things to triplets.” “all of this that’s bloomed between us never ends.” “I want you to kill for us.” “I think you don’t know what you want.” “Pick a bloody side!” “they’re good for a fling, but they’re not relationship material.” “We’re done.” ”Your misadventure with the older Winchester was quite the topic of speculation.” “you run off with your boy toy, Dean Winchester”
You let it simmer.
You bring in the idea of “shipping.” You give them three variations of the word “Destiel” to ponder on and a love song from “Cas” to match. “They’re a couple in real life.”
You let it simmer.
You give them “Bromance” Let them sit with the idea. Was it bromance? Or could they leave the “b” out of it?
You give them “There is no us.” You give them Dean choosing Castiel. You place Castiel in the spot of Cain’s romantic interest.
You let it simmer.