Sirius thought about Snape a lot in Azkaban. More than he would have been comfortable admitting. But it was easy to do, with his hate being as pure as it was. Hate was something the Dementors didn't pay much mind. His thoughts of Snape were his own, and he could be assured of being left alone when dwelling in them.
It wasn't a problem when Sirius thought he would never get out.
Afterwards, though, when he was out and sane and cognizant of the fact that a huge chunk of his sanity was owed to Snape's presence in his life…well, that was problematic.
Sirius tried ignoring him. After all, Snape didn't enter into the equation. Harry did, Molly and Arthur's little boy did. The little boy's rat most definitely did. Snape did not.
After nearly twelve years, though, it seemed Sirius had lost his ability to focus doggedly on one problem. The quiet in Remus's demeanor that Sirius would catch a glimpse of when the erstwhile professor took walks on his own was calming. The echo of James that was Harry when he was airborne was captivating. Snape's all too familiar threats and snarls were maddening. Tired of fearing every emotion but anger and sadness, Sirius allowed the feelings to gush through him, often without care that he would end up shaking on the floor of the forest later, overcome by sensations that his mind could no longer process.
Hence, ignoring Snape was as impossible as it had been when they were sixteen. At thirty-three, Sirius was beginning to wonder how, exactly, the boundaries of hatred were defined.
Sirius found it pretty evident that Snape was still clear on what hatred meant. Hatred meant turning someone over to Dementors. Even more so considering that Snape knew what it meant to be at their mercy, day in and day out, until it was hard to understand the difference between their presence and their absence.
If it had been Remus trying to turn him in, he would have understood. Remus, who he'd betrayed in a moment of adolescent foolishness. If it had been Jamie…well, he would have gone to the Dementors without Jamie having to ask. But of people he owed things to, though Snape was most certainly on the list, he was considerably lower down.
Sirius was pretty sure he didn't owe Snape his soul.
He got to telling Snape this two years after he'd first realized it, caught in a storm of house-bound claustrophobia and short temperedness, "You think you're owed things you have no right to."
Snape shot back, "And you think yourself free of consequences regardless of your actions."
Sirius rubbed at his eyes in exhaustion. "What do you want from me?"
It dawned on Sirius, when Snape didn't answer, that perhaps his notions of hatred were no more clear than Sirus' own.
"Say you're sorry. And mean it," Snape amended.
Sirius hadn't really gotten past the need in his life to one-up the other person, so he started, "I'm sorry I called you a long-haired pansy on the train."
Snape started to interrupt but Sirius just continued, "I'm sorry I substituted powdered bat nails for powdered goat horn in your Lucidity Potion and lost you points as well as respect. I'm sorry I tripped you on the way up to the Astronomy Tower and caused you to fall down the stairs. I'm sorry I created the nickname Snivellus. I'm sorry I egged James on to hoist you up over a tree and strip you. I'm sorry I laughed at you. I am most especially sorry that I told you something that could have meant grievous harm and possible death to you. I'm sorry."
Snape frowned. "Apology accepted."
He knew he wasn't supposed to leave the house, he knew it, but Snape had lied and that got under his skin like none of Kreacher's taunts could. He slipped out of the place, just a big, black stray making his way through London until he was somewhere away from the crowds, somewhere safe to Apparate. He felt slightly guilty for nabbing Remus's wand to do the trick, but desperate times…
The passage through the shack hadn't been closed up so as to allow Order members passage through it. Padfoot took it back into the school, never once going above ground level, prowling the dungeons long enough to find Snape's rooms. He barked loudly, constantly. It only took a few minutes for Snape to give in, growling, "Everyone in the school will hear you, you bloody imbecile."
Sirius figured that since he'd been playing the bloody imbecile by prowling through London, barking at Snape's door was hardly going to make anything worse.
"What if Umbridge comes asking about the disturbance? She's got Malfoy and half the others reporting to her, you know?"
"I'm sure you'll come up with something. A new familiar, perhaps? After all, isn't that what you do for the Order, come up with stories to tell Voldemort?"
"You know nothing about what I do for the Order." He left it at that. Sirius wished someone would tell him something. It might make him feel less out of the game.
"You said my apology was accepted. You said it."
Snape wrinkled his nose. "It was. Is. What is this about?"
Sirius rolled his eyes. "What do you think?"
"Ah, talking it up, is Potter? The arrogant brat stole into my Pensieve." Snape enunciated each word with unerring precision. "Do you have any idea the-"
"I do have some experience with having my memories pillaged," Sirius pointed out, "yes."
"Then you should-"
"He's a child, Snape. A curious, sometimes foolish child whose world has now been turned upside down because he's been forced to sympathize with you rather than his father."
Snape's gaze sharpened. "I'm sure."
Sirius sighed. "Hide in your bitterness, I don't care."
Sirius turned to leave, stilling when Snape admitted, "It's all I have anymore."
His back to Snape, Sirius asked, "Do you even want to change that?"
After a silence so long Sirius nearly turned to check that Snape was still there, Snape ground out, "You've made the possibility somewhat alluring."
Ever so carefully, as though approaching a starving bear, Sirius came closer, the tips of his fingers just barely making contact with Snape's all-too-sharp cheekbones. "I could do better."
Snape didn't look much more confident than Sirius felt.
Snape slouched into 12 Grimmauld Place the second night after Remus had left for a mission. Sirius had lit every candle in the place and was busy fighting the urge to slip into Padfoot when he showed up. Unsure if Snape was real or if he had started having claustrophobia induced delusion, Sirius rubbed his eyes and tried, "There's butterbeer in the cellar."
Snape made a face. "Pumpkin juice?"
Which didn't help Sirius at all in his decision as to whether the man standing in front of him was real or not. "Um, sure. How'd you get away?"
"Had to run an errand anyway, thought I'd- I wasn't ready to talk to Albus."
"Ah." Sirius poured a glass of pumpkin juice and handed it to Snape, who cooled it quickly with a Charm and drained it. Sirius offered, "'Nother glass?"
"No, I'm, thank you."
Sirius scuffed his toe against the wood floor. "It was easier talking when we had ready insults."
"I assure you, I can-"
"I was just saying."
Snape nodded. "Since we're not speaking…"
Sirius's gaze sharpened. A smile slowly took hold of his features. "Sure."
He'd had some pretty good times hurling insults at Snape. Kissing him into a blind stupor before a little mutual wanking turned out to be ten times better.
Late in the night, when there was nothing but darkness and no need for anything but honesty between them, Snape prophesized, "It won’t be long now, until you can leave here. Things are changing, they'll keep changing."
Sirius didn't ask if Snape thought they'd win. He didn't ask how long was long. He didn't ask where he'd go. He just took the words in, let his once-worst enemy fill him with the hope he'd been drained of for a lifetime.