The way the whole thing starts is pretty embarrassing. Jake inevitably lets it slip to Dave that his counterpart in your universe was a successful movie mogul and when Dave in turn asks you about it you don't think twice about showing him your entire collection of SBAJ movies and related memorabilia.
It's only later, when you're sitting in the dark with Dave and a few other interested parties to marathon the movies that you remember to feel mortified. You know logically that Dave's not really your Bro, but that doesn't stop every anxiety you've ever had about what your brother/ecto-dad would think of you and would he like you and would he be proud from resurfacing all at once as a huge ball of anxiety in your stomach.
Given the opportunity to make your first impressions however you want to, you, like a total dweeb, spring on him the evidence of your childhood fanatical devotion. It's something you might have thought would actually be a good idea when you were thirteen, but you are cooler than this now. Except that he doesn't know that and now you've fucking blown it (which is pretty par the course for you lately).
You get so wound about it that when Vantas makes some stupid comment about the movie that you'd usually be able to let go, you end up sniping back at him until you're embroiled in a full on argument.
Somehow this devolves further until you're throwing popcorn at each other and you both get kicked out so the others can watch the movie in peace.
From that point on everybody refuses to have movie nights with either of you.
So you end up having them with each other.
At first, you're not really anything to each other and movie night doesn't come all that frequently. So maybe it's more accurate to say that the start comes later - like the first time that you have to endure one of Vantas' memos.
Initially it's kind of amusing, if a little tiresome. You're an old hand at half paying attention to messages from shout-y assholes.
You're half wondering if anybody would notice you taking a nap as long as your shades are on when Vantas' future self shows up in the chat to pick a fight. There's a small commotion in the memo from those who haven't long since left - mostly some predictable jeering. Harley's making a valiant effort to break up the escalating argument, but she's going mostly ignored.
Generally you'd be up for a good opportunity to needle Vantas, but something about this whole situation is making you feel twitchy and uncomfortable.
And then the image of cracking glass surfaces in your mind with startling clarity.
Before you know it, you've grabbed Vantas by the wrist and are dragging him off, tuning out the steady stream of angry shouting with practiced ease.
You sit him down and make him watch a movie with you.
This isn't the last time that you witness a Karkat vs. Karkat flame war. It's also not the last time that you interrupt in the middle by dragging him off. It becomes something of a tradition. It actually kind of surprises you how long it takes him to call you on it.
"Ok, Strider, what the fuck is up with you?"
"Every time you start arguing with future you or past you or whatever, I am going to make you watch a shitty movie with me," you tell him, not bothering to look away from where Ben Affleck is doing something improbable in a stupid-looking red outfit, "I will do this every time. I will also choose movies that are progressively shittier until the universe decides that doing so is an impossibility and we reach maximum cinematographic shittiness. And I will argue with you for every minute of it."
You aspire to be a man of your word.
You're surprised the first time that two Karkats cease arguing without your prompting. You figure he must have finally gotten sick of the routine and try to ignore any feelings of vague disappointment.
He surprises you again when he takes you by the wrist and tells you that it's his turn to pick the movie.
Somewhere along the way you stop grasping wrists and start holding hands and personal space on the couch becomes a non-issue (this backfires once when you fall asleep on his shoulder and he categorically refuses to promise never to speak of it when you ask).
Once upon a time regular old human romance had barely been a possibility, never mind exploration of the troll quadrant system.
You're kind of startled to find you've settled into something strange and warm and comfortable that wasn't according to plan at all.
He tells you that this isn't the way this is supposed to work.
In response you promise to jump on the roof of a moving vehicle and make some pretentious speech about feelings like Troll Will Smith.
You somehow manage to keep a straight face while he struggles between praising you for realizing that that movie was largely about auspisticeship and berating you for thinking that Troll Will Smith and Troll Eva Mendes could have possibly been intended as anything other than an ideal red pair.
He ends up forcing you to watch the film again while he gives you a play-by-play explanation of the characters' various romantic entanglements. It's an even worse movie the second time around, if that's possible.
You can't say that you really mind.