Remus not telling Dumbledore about Padfoot fascinates me. the layers that are there. Remus can’t truly believe Sirius wants to kill Harry, otherwise he’d never let this dangerous advantage he has go unchecked. when did Remus start questioning that idea? when he first read it in the papers? when he came to hogwarts? or is it just another line of the questions he’s been wrestling with since 1981?

then there’s the implication of protecting Sirius. Remus has nothing to lose by exposing Padfoot, aside from his old headmaster knowing he kept a secret in school. and, really, that’s absolutely irrelevant. a poor justification on Remus’s part to Harry (and possibly himself). the person who does have something to lose is Sirius. Remus knows that if he reveals Sirius’s disguise then he’ll eventually be captured and subject to the Dementor’s Kiss. we know from canon he’s at the very least not comfortable with that idea, but his passive inaction becomes an action itself when there’s an intention to protect Sirius behind it

how much does Remus admit that to himself? how does he reconcile that lingering urge to protect Sirius with his belief in his guilt? is that when that belief begins to unravel?

this is also one of the few times (possibly the only time?) we see Remus disagree with and disobey Dumbledore. he’s clearly made his own decisions about Sirius’s guilt and what kind of protection Harry needs, and that doesn’t include stopping Sirius from coming into the castle. he’s working against Dumbledore, especially when he finds out Sirius can and has broken into the castle as Padfoot. it’s so deliciously fascinating how all of this, all of the questions and the doubts and the lingering loyalty, are what push Remus to challenge someone he never has and never will again.

relatedly - assuming Sirius knows Remus is teaching at hogwarts, he’d be aware that Remus is the only one who knows his secret, and if Remus still believes the story the ministry told about him, then he’d likely make everyone aware of Padfoot’s existence. so Sirius is taking a risk in sneaking into the castle as Padfoot just as much as in his human form, because every portrait and coat of armour could be on the lookout for a large black dog. he’d obviously risk anything for Harry, and there’s an argument a dog is less visible than a man, but there’s still part of Sirius that risks trusting that Remus has kept his secret