Sirius Black and James Potter were open books. Very open books. Books that would in fact refuse to close unless you jumped on them, belted them shut and promised to hex them into the next century unless they would just stay shut please. Peter Pettigrew was a nice book. A book that opened when you told it to and closed when you told it to. No promises of hexes or jumping were required. Remus Lupin was a closed book. A very closed book. One that required a sledgehammer, numerous opening charms and the threat of no more reading in order to crack it open even slightly.
If James was hungry, Sirius knew. Peter knew. And if anyone else happened to be around, chances were they knew too. When Peter was hungry, if you were around, you knew, if not then he took himself off to the kitchen and told the house elves. Sometimes Sirius knew, sometimes he didn't, simple as that. When Remus was hungry, Sirius wasn't entirely sure if Remus even knew. You had to escort the boy to lunch then drag him away from whatever book he was reading or homework assignment he claimed must be done right now and make sure he ate dinner.
When James fell for Lily, they all knew. Lily knew. And thanks to James's many proclamations of love, Sirius was pretty sure the entire school knew. When Peter . . . . . well, Peter and love didn't really mix too often. At least not that Sirius knew so maybe that wasn't really the best example. But when Peter had crushes the Marauders totally knew. Yet in all their years at school, Remus had never mentioned a single love or crush or even slight interest in anyone. And Sirius didn't know. But Sirius hadn't spent all those years trying to crack open Remus Lupin for nothing. Someday, he would learn to read Remus.