The actress got fake teeth for her part and kept them after the series ended since they only fit her. She apparently busts them out whenever she wants to bring Bellatrix to life.
Rowling said she frequently saw crying girls in bathrooms, “especially at the parties and discos of my youth. This does not seem to happen in male bathrooms, so I enjoyed placing Harry and Ron in such uncomfortable and unfamiliar territory in Harry Potter and the Chamber of Secrets and Harry Potter and the Half-Blood Prince.”