Disclaimer: I don't own James, Sirius, Remus, Peter or any of the other recognisable characters from Harry Potter. I'm only borrowing them.
This takes place after James, Sirius (and Peter) have discovered Remus's secret. This is my take on how they decide to become Anamagi.
'I've been thinking,' James said, sitting beside Sirius.
Sirius groaned. 'C'mon man. When are you going to get over her?'
James hit his best friend in the arm. 'Never. And I wasn't thinking about Evans. I was solving the problem we ran into while planning ways to stick with Remus when he…goes to see his mother.'
Sirius nodded. 'Okay. What've you got?'
'Anamagi.'
Sirius's eyes widened. 'You're kidding.'
'Nope. Come on, how hard could it be?' James's grin faded. 'Look, we told Remus we're his best friend, and best friends stick together. Think about it, werewolves are only a danger to people. If we became animals…'
'Remus won't be a danger to us,' Sirius finished, quietly.
'Exactly.' James's grin returned.
'We'll have to talk to McGonagall,' Sirius said, a plan already forming in his mind. 'And see if the library has any books on the subject.'
***
The two were still deep in discussion when Remus returned from the hospital wing a few hours later.
'You're both insane,' he said and James and Sirius told him their plan. 'It's too dangerous.'
'Which part?' Sirius asked. 'Getting the ability to change into animals at will, or keeping you company during what you have said yourself is the most difficult part of your life.'
'The whole thing!' Remus said almost too loudly. 'And what if something goes wrong? You have no idea what I'm like when I transform. What if-'
'What if nothing,' James said, cutting Remus off. 'Remus, we know what we're doing. We're going to talk to McGonagall-'
'You're going to tell McGonagall-'
'If you'd just let me finish,' James said, interrupting the werewolf. 'We're going to get an overview of what is involved in becoming an Anamagus.'
'What, you'll just ask her about Anamagi? Don't you think that'll be a little obvious? Unless you were going to tell her your insane plan.'
'No way!' Sirius said. 'We did Anamagi the other day, while you were away. The three of us can go see her and you can ask her about it. Then, in a sudden stroke of inspiration, James and I can ask her a few more questions, to make our essays better.'
'But we won't ask for details,' James continued. 'We can go to the library for that.'
'That's where Peter is,' Sirius added. 'Looking for books.'
'Why aren't you two looking for your own books?' Remus asked.
'We got a month ban after James repleaced the word "and" in every book with-' Sirus started, but Remus held up a hand.
'I don't want to know,' he said, pulling James's Potions notes towards him.
'You helped,' James said to Sirius. 'You were supposed to be keeping an eye out for Pince.'
'Hey, can I help it if your spelling is atrocious?' Sirius argued. 'If it was up to you, every other word in the book would be…'
Remus tuned out of his best friends' argument. He knew he would never be able to sway the two from their ridiculous plan, which meant he would have to help.
