u know those 'marauders act like their animagus forms' hcs?
it takes a while for the characteristics to really show up; becuz the wizard has to be fully attuned to their new form---and there is something very strange of ur body changing forms, even if its just temporary/reversable
and it starts off with small stuff, just little habits or things that can be brushed off
but as they get more comfortable, the animagus form traits become more prominent (until they learn to control them)
longwinded way of saying, that in the marauders' 5th year, they successfully become animagi, but it isnt until 6th year that they start acting like a stag, dog, rat, respectively,
sirius has long since left home; his brother seems to hate him, he claims to hate his brother too
and he sneers or glares when he sees him, trying to mask the hurt when regulus just straight up ignores him---but, though he'll never admit it to anyone, a part of him is still happy, relieved to see regulus. an angry regulus who pretends that sirius doesnt exist is still better than a dead regulus or a hurt regulus
padfoot hasnt gotten the message that theyre (as in sirius and padfoot) no longer on speaking terms with regulus
and so after sirius hasnt spotted regulus in like a week, he sees him walking alone, and before he can school his features, his mouth opens and he starts speaking
sirius: REGULUS??? REGULUS!! OH MY GOD IT'S REGULUS!!! I HAVENT SEEN YOU IN FOREVER!!! HI !! HI HI HI HI HI HI !! YAY REGULUS
he's practically jumping where he stands
while regulus is just standing there like:🧍
the first time, regulus confusedly says hi back and leaves, thinking sirius is making fun of him, and sirius gets his senses back when regulus isnt there
but then it happens again, and again
and if its a joke, regulus is not seeing whats funny for either of them becuz ppl side eye sirius whenever he does it
gradually, regulus starts expecting it, is a bit pleased by it, will give sirius an almost shy smile, wont tense up, ignore, or roll his eyes
sirius/padfoot takes it as permission
now regulus is greeted with the same level of enthusiasm and a giant feet-lifting-off-the-ground hug