"…Longest" by InFabula
Disclaimer: these wonderful characters belong to JK: I just borrowed them for a bit.
Laughter. Not at James, glossy-coated and fierce-antlered. Nor at the huge, shaggy-haired dog that is Sirius. The laughter is just for him.
Insignificant. Little. Vermin. He glimpses how he is viewed and it starts a chill in his heart.
When they show Remus, he has the grace to sit in silent awe. Even so, he imagines laughter later.
"You bring me information beyond price," says the Dark Lord. "Your value to me is equally immeasurable."
Secrets learned at doors; in rooms; under tables. Thoughts exchanged in perceived privacy make tales to be told.
The others know someone is talking. Suspicion haunts their faces. Sirius looks at Remus with new eyes. Remus reciprocates. James no longer jokes.
On his own, he thinks of this and smiles then breaks into protracted glee. Marauder to traitor: the change is complete.
A/N: This was submitted via the Sugar Quill as an entry in the fan fic part of the Phoenix Rising Competition where you had to choose a quality associated with a phoenix - I chose transformation - and use it as the basis of your story: I was thrilled to have an honourable mention.
PS The title is not an ironic reference to the length of the story: it's from the saying "He who laughs last, laughs longest".
PPS I am aware that the author's note has just about doubled the length of the story :-)