Sirius couldn't find his friends and he was bored, so it was a very good thing that, when he walked into the bedroom, Remus was there. He was less pleased that Remus seemed thoroughly interested in a book.
Sirius was not a big fan of this "reading for pleasure" thing. He'd never met a book he liked, although he'd read plenty because it was rather delightful to argue with people about whether Elizabeth and Darcy's marriage would last (hell no); in his opinion, there were far, far better things to do with one's time.
Or at the very least one could read in the common room so people didn't have to go all over the school looking for you, honestly, Remus.
Sirius's usual mode of entering the room was to fling himself at the person he wanted and hold them down until they caved, but Remus would be upset if he wrinkled the book, so he sat down rather normally instead and reached over to bat at the page, just to make sure his friend paid attention to him. And just in case his motive was not clear, he added petulantly, "Moony, pay attention to me."