Do you have any random headcanons about Severus? Something less based on canon and more on your personal interpretation of the character?
No canon-related? Just vibes-based? Yeah, I've got quite a few:
He has chronic, undiagnosed depression, which basically explains why he doesn't give a flying fuck about anything that doesn't require an actual compelling reason for him to be invested. It also explains why he still lives in his childhood home even though the neighbourhood is awful and the house itself is full of horrible memories. It explains why he's got such a shitty personality, too. I've also always found him vaguely suicidal in general, not in the sense that he's actively trying to die, but more like he doesn't particularly care whether he lives or not. He's just there, purely out of a sense of atoning for his sins.
He absolutely despises Quidditch. Like, genuinely cannot stand it because he finds it mind-numbingly boring. That said, he likes it when Slytherin wins purely out of house pride.
Tobias used to force him to watch football matches as a kid, so he understands the rules, knows how the game works, and all that. But it's the same as with Quidditch: he finds watching sports unbearably dull.
He started smoking at around thirteen because, in the area he grew up in, it was pretty common for kids to pick up the habit ridiculously young. He regularly stole cigarettes from his father.
He always had to fend for himself when it came to cooking because Eileen either didn't cook much due to being deeply depressed, or because she simply wasn't very good at it. So he learned how to cook at a fairly young age. The ironic part is that despite knowing exactly what he's supposed to do, he's absolutely terrible at it. He's a genius when it comes to following potion recipes, but making edible food is somehow beyond him.
He doesn't particularly enjoy eating in general. He does it because otherwise he'd die, basically.
If you asked him to choose an alcoholic drink, he'd tell you it's wine. But the one he can actually drink most easily is beer, even though he tends to avoid it because it reminds him too much of his father.
He's exactly the sort of man who would tell you he doesn't need therapy and that the last thing he needs is to discuss his problems with a stranger, because back in his day people sorted their own shit out and didn't need any of this modern nonsense. Above all else, Severus is a Boomer trapped in another generation, and he thinks like one.
Spoiler: he needs more therapy than your dad.
He has a great deal of insecurity when it comes to relationships and interpersonal connections, along with very low self-esteem regarding both those relationships and his physical appearance. Intellectually, however, he knows he's a ten out of ten. He doesn't doubt for a second that he possesses exceptional intelligence and talent. If there's one thing he has in abundance, it's intellectual arrogance, contradictory as that may seem. But that's because he's built what little self-confidence he has around that aspect of himself.
This is a headcanon I bring up all the time because I love it far too much: when he joined the Death Eaters, Narcissa was basically trying to find him a girlfriend every five minutes. She kept attempting to set him up on blind dates in a very older-sister sort of way, but Severus was an absolute disaster and often didn't even show up. He only agreed to go to one or two out of deference to Lucius, but they were among the worst experiences of his life. He had never felt such intense second-hand embarrassment.
He has absolutely no idea what to do with a baby. He's one of those people who becomes incredibly tense the moment you hand one to him because the only thing he can think of is people making silly faces and using ridiculous baby voices, and he would Avada Kedavra himself before willingly engaging in that sort of behaviour. He and babies are the worst combination imaginable.
He never puts sugar in anything. Not in tea, not in coffee. He prefers bitter flavours.
His sense of humour is atrocious. Not dark, beyond dark. Pitch-black. Full of deeply uncomfortable references. In fact, he doesn't openly express even half of the things that occur to him. He knows very few people around him would actually appreciate that kind of humour, not even among the Death Eaters. But every now and then he'll come up with some truly horrific remark in his head and find himself absolutely hilarious. He'll quietly laugh to himself because, deep down, he thinks he's incredibly funny. And yes, those thoughts are usually related to the person standing in front of him and how spectacularly unintelligent he considers them to be.
He prefers cold climates to warm ones, and the very concept of going to the beach is one of his worst nightmares. It could probably become a boggart, honestly.
Going to London gets on his nerves. The hustle and bustle of a big city irritates him to no end.
He's surprisingly charismatic when drunk. The problem is that it's surprisingly difficult to get him drunk in the first place, even if he drinks a considerable amount, because he tolerates alcohol like a bloody Viking.
He's unbelievably stingy, to the point where it genuinely warrants professional evaluation. He won't buy new clothes unless the old ones are literally falling apart. It stems from having had nothing as a child. Even if he had become incredibly wealthy, he wouldn't spend a penny unless it was absolutely necessary. Any self-indulgent purchase takes him days of deliberation because once he actually buys it, he feels genuinely guilty about it.
In keeping with that, he's the type of person who insists on splitting a bill down to the very last penny.
He's an incredibly proud man. He'll never admit that he needs help or that he's struggling because he insists on handling everything himself. The idea of depending on other people makes him deeply uncomfortable.
Growing up in Spinner's End taught him that the best defence is attack. When he got into fights in the neighbourhood, Tobias forced him to physically fight back against whoever had gone after him. By the time he arrived at Hogwarts, creating spells simply seemed like the logical equivalent because that's how people attacked each other there. The law of the street still applied.
He would rather drive an enchanted car like Arthur Weasley's than get on a bloody broomstick.