I'm actually very nervous about writing already-existing characters. Worried I'll fuck it up. That's why a lot of my old fics were OC-centric with mention of original cast. A friend asked me to roleplay Amy the Hedgehog one time and i had a mild panic. Not an attack, really. Just a minute or two of 'No". Might be why what got me into testing the waters with original cast stuff were zombie novels. They had pretty shallow characterization to begin with.