A/N Here is my take on why remus so easily beleived Sirius that night in the Shreiking Shack. I have seen it done before but always in Azkaban. I personally think it was more likely to happen during the Prisoner Of Azkaban Novel. Anyway, hope you enjoy!
The Hope of a Convicted Man
The Best Marauder
Remus sat in his office staring at the Grindylow as it floated effortlessly in its small tank. The Professor was stuck in the dark recesses of his mind thinking about his former best friend and lover Sirius Black. Ever since he had escaped, Remus' thoughts turned more and more often to the canine like man. Usually the thoughts were bittersweet-Remus remembering everything he had had and everything he had lost-but today they had taken a more sinister turn.
Remus was wondering how Sirius had made it into the castle and-even though he wished he could deny this thought-whether the man was still here. Did he want to speak to Remus? Or did he hate the werewolf because he hadn't tried to help him escape Azkaban, let alone demand for him a trial?
The Professor escaped the dark workings of his mind and looked up as he heard the door creak open. Iit really did need to be oiled Remus thought. A dirty man entered the room, back to Remus, meaning he was unable to identify the intruder. The man closed the door slowly, trying to quieten the creaking and turned towards his prematurely aged companion. Remus started as he realised who it was and his jaw temporary lost the ability to work causing his mouth to drop open.
"Moony!" the man said happily, "How've you been? I mean, apart from the obvious distraught and what not." Sirius finished on a decidedly less cheerful note.
Sirius watched as Remus tried to speak but his jaw was yet to work out it was capable of movement.
This caused Sirius to grin. Then he noticed the look in the other man's eyes-hidden behind the shock and confusion Sirius could see Remus' eyes churning with anger. "Moony?" the slightly shorter man questioned carefully.
"No!" Remus replied, finally gaining back the control of his mouth.
"No!" Remus said again, "Don't come in here and 'Moony' me and expect things to just go back to the way they were before everything happened!" He stood up and glared at the man in front of him making sure the other realised just how much shit he was in.
"Moon…er, I mean, Remus?" replied Sirius catching himself before he said the nickname again.
"No, Sirius!" Remus said in his deathly calm voice that had always scared the other man. the professors hands moved to his pockets and formed fists. Remus was having a difficult time trying to fight the urge to break the other mans nose. "Don't. You shouldn't be here. You should still be in Azkaban."
Sirius flinched, obviously hurt by Remus' words. "You can't really believe I deserve to be there, that I killed them? Can you?" Sirius asked in a small voice. He shrank in on himself, trying out his puppy dog eyes. Sirius seemed to have trouble pulling his face into the right shape, he really needed to practice.
"Of course, I do! Who else would have done it Sirius?" Remus anger and hurt was beginning to slip through into his words, something Sirius had only seen happen once before. He took a step back, his back almost against the door.
"P-Peter-Wormtail-he did it."
"Ha! Funny Sirius! Nice try, you never liked being blamed for anything but this time all the evidence points to you. You were the secret keeper! You were the one who betrayed them to Voldemort! You were the one that survived the street being blown up! Not Peter." The werewolf's voice gradually grew louder throughout his speech before coming back to that creepy calm on the last two words.
Sirius was-he couldn't deny it-scared, Remus didn't shout. Ever. "Remus," Sirius managed to choke out, "James was my brother, apart from you he was the most important person in my world. I would never do anything to hurt him." Sirius stopped to draw in a ragged breath, running his hand through the filthy, matted hair that had once been his pride and joy.
He continued in a calmer voice, "Peter and I, we switched. He became the secret keeper; after all who would expect him to be trusted with such an important thing? I thought it was a brilliant idea. No one woulde beleive him anyway and it meant I was still able to go on Order missions without having to worry about giving away the secret. I couldn't tell anyone if I didn't know, right? And then Peter told Voldemort, he turned spy and told him! How was I to know he was the traitor, a snivelling coward like him? And then I went after him and he blew up twelve innocent people, managed to escape and make me look like the murderer. Now he has come here, to Hogwarts, and I intend to finish what I started. I will kill him."
Remus snorted, "Azkaban really does send people loopy."
"You're the loopy one here, Lupin, I'm just a serious black."
Remus groaned at the terrible joke, making him think of his teenage years and all the fun they had had together. He quickly snapped back to the present though, remembering just who was standing in his office. "Get out, Sirius. Before I tell Dumbledore that you're here." Remus was cracking and he knew it, never could he stay angry at Sirius long.
Sirius noticed too and walked closer to the aged man. He froze, though, as he heard three voices outside discussing loudly whether Professor Lupin would want to see them.
The man covered in grime quickly stepped forward, grabbed the front of Remus coat and pulled him closer so there noises were about two inches apart. "This is going to sound desperate but what the hell! Remus, I love you, always have, always will but I need you to believe me when I say I'm innocent. I don't think I can stand the thought of you thinking I killed Lily and James. I loved them and there's not a day that goes by where I don't regret what I did. What I did to you all. I'm sorry for leaving you alone for thirteen years and I'm sorry that I suspected you as the traitor but, Rem, I love you and I don't want you to hate me." Sirius said it all as quickly and quietly as he could manage, hoping to show just how much he meant it all by his solemn expression. When Remus didn't pull away, he leant forward and pressed his mouth against the taller man's, holding it for what he considered too short a moment. He pulled back and looked Remus in the eyes one last time before turning and transforming into his animagus form, begging Remus with his eyes not to give him away.
The door flung open and Harry stepped in momentarily shocked by the sight of a dog standing next to the man he wished to speak to. "Umm…Professor, why's there a dog in here?"
Remus shot the dog a stern look before saying, "I was just about to make him leave, I have no idea how he got in here."
"Oh…right." The boy said as the dog walked passed him head down and tail still. Sirius turned back one last time to look pleadingly at the man he loved.
The last thing the large dog heard was, "Now, Harry, what brings you here?" before the door closed behind him with a resounding 'thunk'.
Sirius slowly made his way down to the Whomping Willow upset that Remus had not showed any signs of believing him.