Hiii 😃, how do you think Severus would act if he were in love with someone?
Hey! This depends on a lot of factors, i think, such as what their dynamics is, how this person behaves, and what the external circumstances are, but we can draw some conclusions from how Sev communicates with people he loves/likes in canon, most notably Lily, of course, but also Malfoys, Albus, to an extent some of his collegues such as Minerva and Slughorn, and also from his overall character traits. So here are my headcanons on the matter:)
Severus still can be waspish and rather dry with them, he doesn't lose his sarcasm, but he is less hostile and abrasive. His primary love language is DEFINITELY acts of service, he would exhaust and exploit himslef so that the person he loves has everything they might possibly need, and he would quietly try to make their life easier without showing his efforts off. However, it'd be extremely hard for him to express affection through words, and I can't imagine him initiating physical contact a lot – but i think he'd immensely enjoy it if the other person does. He would try to pay attention to their moods and preferences, too, tho I'm not convinced he'd be infallible with that.
He would defend them against others, verbally and with his actions, even if it puts him in danger or is uncomfortable for him. He would also be inclined to justify all the bullshit this person might do and take their side in any external conflicts – at the very least until they can talk one on one. He'd prefer to keep things more private, but he's also not the type to act as if the person dear to him is a stranger – unless it's what he thinks they want. He'd probably be uncomfortable with public displays of affection, but tolerate them if they are important for his loved one.
If someone is actually dear to him, Snape would be more prone to apologise and claim fault after arguments, or back down during arguments, as we see he had been with Lily. Speaking of arguments, I dont think he's the type to get dramatic, unless the situation is really dire – then he can get rather hysterical and say things he'd really regret, but he needs to be pushed pretty far for that – yet he can get passive agressive and avoidant, or agitated and defiant. I feel like he might be the type to subconsciously try to push that person away emotionally "for their own good", yet try to be in their life as a protective figure rather then an attachment figure, sinse this is something he's used to – unlike genuine emotional closeness. He is not the type to "kill for them", especially not impulsively, he's much more of a "die for them" – or a "live for them".
He'd absolutely not actively pursue that person if they aren't together already, let them know about his feelings and try to get in a relationship with them, because he doesn't think that it will work out, or that he even theoretically deserves for it to work out; but if they already have an established platonic or romantic relationship, he'd hold onto that dynamics for dear life. If the other person doesn't show initiative, the romantic potential of the relationship is basically cooked lol.
I feel like he might fall into codependency and go through with a lot of things he doesn't want or doesn't agree with. There would, however, be some things he'd not drop for anyone, such as his academic passions, or if we are talking about second war/post war Snape – his sense of duty and responsibility. It also makes sense to me that if he feels truly comfortable with someone, he'd get way more relaxed then how we usually see him, more clumsy, less put together and mentally armoured. We never really see Severus feeling fully safe with someone and having trust in another person in canon, after all.
Also, if he already loves someone, the continental plateaus will move on sooner than he does. This man literally doesn't get over it, period. He would leave them alone physically if (he thinks) it's what they want, but their image wouldn't leave him alone mentally.
