They started sleeping together before they did anything else.
Sleep in the literal sense.
One night Dean rolled over and Cas was just there, mouth open and snoring softly, one hand unconsciously reaching in the open space between them.
They didn’t talk about it.
At first it was a middle-of-the-night thing, where one of them would sneak across the hall into the other’s room and crawl into bed without waking the other up. Then they’d wake up in the morning, get up immediately, and go about their day like nothing had happened.
Then it was a conscious decision. They would retire at the same time at night and Cas would reach his hand back and Dean would take it and they would fall into bed together and fall asleep immediately.
Then the cuddling started. Dean woke up one morning with an arm wrapped around his middle, and he pulled on it to make Cas come closer, and they each took a deep breath and melted into each other. They both pretended to be asleep for an hour that morning just so they could keep touching.
The tension got so thick that neither of them could sleep at night, tangling and turning and trying their damnedest to get as close to each other as humanly possible without actually doing anything. Because if they did anything, then they’d have to talk about it.
Three months. It went on for three months until one night it got really bad and there were hands sneaking up under shirts and frantically gripping skin and there were butterfly kisses on collarbones that turned into hickeys on necks and finally, finally Dean breathed, “Kiss me,” and Cas found his lips in the dark and then found them again and again until they fell asleep on top of each other.
Sam said, “Congratulations, guys,” the next morning and when Dean and Cas asked what the hell he was talking about, he explained, “You look well-rested for the first time in months.”