Emma Watson was too diligent
The casting of Harry, Ron, and Hermione was really good. So good, in fact, that the three partly matched the characteristics of their roles. Emma Watson, who played Hermione, wanted to do everything right in her first major acting job. Therefore, she diligently learned not only her own lines but also those of her colleagues with whom she acted in a scene.
However, this led to her not only being able to recite Harry and Ron’s lines by heart but also whispering them along. This becomes clear whenever she moves her mouth but actually says nothing. Director Chris Columbus tried to train her out of this habit, but some scenes with mumbling Hermione still made it into the final film.
How can the mistake happen? In this case, it’s quite self-explanatory. Emma Watson was just TOO diligent and wanted to ensure that her colleagues’ lines were also in place. However, she took it a bit too far.
