Writing a fictional story is tough. It's hard to know how you're going to tell the story if you're just winging it most of the time. Writing fanfiction is tougher; you have to make sure you're telling the story in such a way the characters are all in-character.
Rewriting a written fanfiction from a good friend is easier, but not without challenges of its own. You don't want to give away too much vital information to the story, otherwise you'd give away the story before you write it. At the same time, you have to keep them interested, otherwise you're doing a shit job as a reviseory writer.
...I don't think reviseory is a word. It should be.