Kreacher

According to the original script, the character of Kreacher was not intended to be in Order of the Pheonix at all. But after J.K. Rowling read the script, she more or less insisted on him needing to be there to avoid some serious problems with future installments of the series. Though Kreacher has no noticeable impact on the plot or story as it's presented in the film, a couple of scenes with him were added at the last minute based on this request.