"Jimmy Kimmel Live."

Watson was on the late show to promote "Beauty and the Beast," but she'll never stop talking about Harry Potter. She was a huge fan of the books before the film franchise even started, and she ended up bringing her adorkable fan fawning into scenes ... which she would then ruin by mouthing her co-stars's lines.

Kimmel played a clip of the young star mouthing lines through a scene with Daniel Radcliffe's Harry Potter and Rupert Grint's Ron Weasley. Kimmel laughed, but Watson hid her face in embarrassment:

"This is actually quite traumatic for me because I created issues because of this. I would ruin takes. [Director] Chris [Columbus] would be like, 'Cut, Emma. You're doing it again. You're mouthing Dan's lines.' I'd be like 'Ooh! I'm so sorry! So sorry! I feel so bad.' But I couldn't help myself. It was like, I was such a loser. I really love those books. I really wanted to do my job well, and I kind of overdid it."

Jimmy said Guillermo does that to him all the time, but you can't tell because of his mustache. So if young Emma Watson had only had a mustache, that would've solved everything.

Watch that video, and a few more from her time on JKL:"Beauty and the Beast" is out in theaters next Friday, March 17.

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