Helena Bonham Carter did play Hermione in the last film, as Hermione uses polyjuice potion to assume Bellatrix' shape in order to fool the Goblins at Gringotts. Helena knew of course that she had to do things as Emma Watson did, and that must have worried her. I've been told Carter did visit Watson in her own house in order to discuss properly how Watson played Hermione in order to make the viewer still feel they were seeing Hermione, even though she looked like Bellatrix.
This gif must be a fragment of an interview about this particular part of the movie.