Fan creativity is as old as storytelling. Distribution is a lot wider these days, though. If you want a live singalong of Once More with Feeling you may need to inquire about rights. (…) Legal concepts of transformativeness have broadened over the past 15 years, but there’s still a lot of confusion and paranoia—in part because fanworks are created by 12-year-olds and 90-year-olds with different levels of knowledge.
Heidi Tandy paraphrased by Rebecca Tushnet, Penn symposium: fan fiction