James Potter was an easy choice for the Sorting Hat. His bravery was obvious. He grew up brave, with the Potter name, and was thus always brave. It was who he was. He stood in front of the Dark Lord and fought without a wand, cast down with a single, awful curse. His bravery was physical and brash. He defied the darkest wizard alive.
Remus Lupin, who was given an awful curse before he even turned five, was easy too. His monthly struggles showed quiet, unseen bravery that radiated in his blood, even if he never saw it. He never cracked under the pressure of friends that he finally had, school, the inevitable pranking that he got involved in, and monthly transformation. He survived 12 years in transformations after 10 years of friends with him, helping him through the process. His bravery was quiet and unparalleled. He defied his nature.
Sirius Black was harder to sort. His whole family was Slytherin. He showed some of these Slytherin qualities, qualities his family beat into him from a young age. He was sorted into the rival house, certainly shocking everyone, and angering his family. He never looked back to the Slytherin table as he nervously walked to the Gryffindor Table, slowly with eyes on him. He quietly sat down by himself, and waited for someone to talk to. His bravery was under appreciated and strong. He defied his family.
Finally, Peter Pettigrew took the longest. He was nervous, quiet, and twitchy. He was sorted into Gryffindor because he would later help and stand by a werewolf to become an animagus for him. He also betrayed his friends, a twisted sort of bravery to assist in murder of a best friend and condemn another to prison for over a decade. To take away the innocence and lives of three young men, just 21. His bravery was twisted and forgotten. He defied his friends.