It turns out that Emma Watson had one bad habit that risked ruining the Harry Potter films when she was younger. During rehearsals, Emma could not help but whisper her two young co-stars' (Daniel Radcliffe and Rupert Grint) lines.The host pointed out this quirk, which is not at all visible in the films, and the actress happily explained herself. She said this bad habit was "pretty traumatic". She also said:"It caused some problems on the set, I screwed up a lot of shots because of it. Chris (Columbus, the director) had no choice but to intervene: 'Cut! Emma, you're doing it again. You're mumbling Dan's lines.' But I couldn't help it. I was such a loser. I loved the books and I wanted to do my best, but I actually did too much!"Judging from her career, the actress, who has since worked with more huge stars including Sofia Coppola and Darren Aronofsky, doesn't seem to have been affected by this at all.Take a look at the video above for more!