A non-exhaustive list of random Sambastian headcanons I have:
Sam loves to play the role of resident funnyman and is always joking around. A big running bit in their relationship is Sam trying to rile Sebastian up with increasingly stupid jokes, or by saying really unhinged, out-of-pocket shit, to which Sebastian chronically underreacts. Most of the time, Sebastian will just ignore his antics or quietly raise an eyebrow, and occasionally he'll make some dry, sarcastic comment in response. It is SO hard to get a genuine laugh or a rise out of him, and Sam feels like he's won when he does.
In a similar vein, Sam loves making a game out of everything. He is the epitome of "what would you give me if I can get this wrapper into the trash from here?". The phrase "I dare you to" is like a sleeper agent code to him, and Sebastian derives a lot of silent pleasure from watching Sam do dumb shit that he knows will fail (within reason).
Sebastian does, however, often have to rein Sam in. Sam will say yes to pretty much anything, so Sebastian generally has to be the voice of reason. He'll be the first one to pipe up if he thinks someone is taking advantage of Sam or pushing him too far, and he has zero trouble watering down his words.
When it comes to harmless pranks though? Sebastian is 1000% enabling him. Sebastian is very anti-establishment and Sam has a real mischievous streak, plus they have to work hard to engineer fun and novelty in Pelican Town, which creates the perfect storm for these two to dabble in delinquency. Most of the time, Sebastian is the brains behind the operation, while Sam is the one who enacts it and subsequently reaps the consequences.
Sam sends a LOT of texts. That can be anything; memes and brainrot, requests to hang out, pictures or videos of things that happened during his day, random thoughts he just had to share. He will send things out of the blue with no context and he WILL pester Sebastian until he responds, as if every single nonsense message is something urgent.
Sam texts with abysmal spelling and zero concept of grammar or capitalisation, and uses a lot of emoticons (a la his little (^_-) on the request billboard). Sebastian is, conversely, the driest texter in the world. 90% of his replies to Sam are a single word, if not just a thumbs-up react, and it often takes him hours or even days to respond.
That said, it's rare that Sam ever just imposes himself on Sebastian without asking. As much as he loves to badger him via text, he knows that Sebastian can reply at his own discretion, so it doesn't feel so much like crossing a boundary. If he wants to hang out, he will almost always text before coming over, unlike pretty much everyone else in Sebastian's life; a la his 2-heart event.
As much as they razz each other, they do both respect each other's boundaries, and would stop if it ever went too far. They're also both very protective of each other. Sebastian can call Sam an idiot, but if anyone else does (excluding maybe Abigail), they're getting socked in the face. Similarly, Sam really hypes Sebastian up when he's not around, and he's the first to rush to Sebastian's defence if anyone says a bad word about him.
They watch a lot of horror movies together. Mostly shitty old ones that they make fun of. Watching bad movies together is a great middle-ground compromise for them, because their media consumption styles don't really align much otherwise. Sebastian is a real snob about film/movies and loves to analyse the shit out of them, while Sam has the attention span of a goldfish, and either has to provide constant commentary to entertain himself or ask clarifying plot questions because he stopped paying attention at a crucial moment. When they watch bad movies, Sam gets to crack jokes throughout the whole thing without Sebastian getting irritated, and Sebastian gets to give his thorough rundown afterwards on all the various reasons why it sucked.
On that note, Sam loves when Sebastian goes on tangents. Sebastian is a notorious overthinker, but most of that thinking is confined to his brain, and never sees the light of day. There are certain specific things that get him fired up enough to ramble, but unlocking those things is incredibly rare and is usually thwarted by his social anxiety. When he gets talking about something he's passionate about, or ranting about a visceral pet peeve, Sam -- yapper of the century and a chronic interrupter -- will immediately shut up and listen. He loves hearing Sebastian's thoughts and admires how eloquently he phrases things.
Oddly enough, it's actually pretty rare to get Sam into that zone too. As much as he loves to talk, it's mostly just spontaneous thoughts or random hypotheticals or bad jokes. He is the ASS trio's biggest punching bag and makes fun of himself a lot too, because he doesn't take himself seriously (and on a subconscious level, has limited faith in his own intelligence & competence, so it's easier to be loud & proud about his flaws than to invest legitimate faith in himself and risk being let down). So when he starts talking earnestly about something he's passionate about, Sebastian is equally in awe.
Sam's primary love language is physical touch, which is true even in friendships. He stands way too close when he talks to people and is no stranger to a pat on the shoulder or a friendly hug. So, long before dating is ever on the table, he's very comfortable initiating casual touches with Sebastian. When they sit together, it's always shoulder to shoulder. He'll throw an arm around Sebastian when they're standing next to each other, or provoke Sebastian into playfighting with him by being a menace. Sebastian is very picky about his personal space and he'll make a show of squirming or rolling his eyes if they're around other people, but in private, he doesn't necessarily mind; Sam is the only person who's even allowed to get close to him without being swatted away.
Transitioning from a friendship to a relationship feels very natural, because a lot of what they end up doing is stuff they would've done before anyway. They don't really go on "dates" so much as just hang out as normal; video games, movie nights, band practice, late night bike rides, weekend trips to the city. I tend to imagine that for a while, they just kind of feel like friends who hook up, and nothing actually changes outside of that, because they're already so comfortable together and their love for each other has run deep for a very long time.
In any universe where they're openly in a relationship, Sam LOVES to be annoying about it. He loves PDA. He loves to give Sebastian this dopey, lovey-dovey look and drop a kiss on his cheek. He loves to use pet names, and will always drop a "babe" or "baby" into conversation every once in a while. He is in love with Sebastian and you better fucking know it.
While I'm at it with the touch headcanons, Sam is a cuddler. He wants to cuddle with Sebastian All. The. Damn. Time. Whether it's properly spooning or just holding Sebastian close to his chest, he doesn't care. He just loves being super close. My image of Sam is >6 inches taller with a lot more muscle mass, so you best believe he will be squeezing Sebastian in an iron grip and never letting go. God forbid he falls asleep and Sebastian needs to pee.
Sam is a very chaotic sleeper. He has a perpetually high body temperature and flails in the night, which is a combination of traits that generally leads to his bedsheets being flung off in his sleep. Sebastian, meanwhile, runs cold and curls himself up into the foetal position during sleep, so he is the sheets hog. This mostly works out, except that Sebastian tends to have a lot of trouble falling asleep and staying asleep, and sharing a bed with someone who kicks him in the middle of the night definitely doesn't help :/
They will typically fall asleep cuddling. If Sam wakes up in the night to find that Sebastian is on the other side of the bed (which is often the case), he will groggily reach for him to correct the distance before immediately dozing off again.
Sebastian's love language leans more towards quality time. He is very receptive to Sam's need for touch, but he won't often initiate it himself. If he loves someone, he shows that just by letting them be in his space. He doesn't mind even if they're sitting in the same room, dead silent, working on completely different things. Having someone in his presence is a huge display of trust in itself.
Sebastian is the practical one in the relationship. He's the one who drives them everywhere. He's the one who pays. When they live together, he becomes the primary cook and cleaner. Sam likes to experiment in the kitchen from time to time, which Sebastian would find adorable if not for the fact that he ALWAYS gets stuck doing the dishes. And Sam somehow makes a million of them. The state of the kitchen counter after he cooks looks like they've hosted a three-course dinner for a party of eight.
Sam is the main proponent of all the cute, romantic shit between them. He's the one who offers his jacket when it's cold, or offers Sebastian a massage when he's tense. He's also really good at pulling Sebastian out of depression spirals and championing a better work-life balance. One of the cons of making his own hours is that Sebastian would willingly stay up until 5am working at his desk, operating on nothing but coffee and a prayer, so Sam will always drag him away before he can wreck himself. He knows exactly how to be convincing ;)
Although, Sebastian working from home & being a reclusive workaholic does have a major benefit for Sam, in that it means Sebastian is home pretty much all the time. Sam needs constant company and attention, so it's nice to have Sebastian around a lot, even if oftentimes, he's not allowed to bother him.
At the end of the day, for how different they are in many respects, they complement each other perfectly. They can communicate whole sentences with just a look. They talk so fondly about each other when they're not around. They both have a love and care for each other that runs very deep, and that persists no matter what form their relationship takes.