She started playing know-it-all Hermione Granger in the Harry Potter film franchise at the tender age of nine.

But Emma Watson's embarrassing childhood habit came back to haunt her when she appeared on Jimmy Kimmel Live!, reports The Daily Mail.

The 26-year-old actress and women's rights activist was mortified when the TV host aired an outtake from the first film, Harry Potter And The Philosopher's Stone, which saw her mouthing along to co-star Rupert Grint's lines.

Emma Watson appears on Jimmy Kimmel Live. Photo/Getty
Emma Watson appears on Jimmy Kimmel Live. Photo/Getty

Watson was left giggling with embarrassment, putting her head in her hands as the clip played for the audience to see.

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"You're helping them with their lines!," Kimmel joked.

The British beauty explained: "You laugh, but this was quite traumatic for me because I created issues because of this! I would ruin takes!

"I would be like, 'I'm so sorry! I feel so bad!' I couldn't help myself."

The star explained that she was such a big fan of JK Rowling's wizarding series that she couldn't help uttering the words from memory.

She laughed: "I was such a loser, I really loved those books, I really wanted to do my job well, and I kind of overdid it..."

The Hollywood favourite also touched upon how lucky she was to land the part that launched her career.

The Beauty And The Beast star enthused: "No one goes for the first audition they've ever gone for in their life - and makes those movies for over a decade."

Watson recently explained why she's decided not to take selfies with fans anymore, revealing it was taking over her life.

Although she's now selective about her interactions, Watson says she's still happy to give people a piece of her time.

Speaking to Vanity Fair magazine, she said: 'I'll say, "I will sit here and answer every single Harry Potter fandom question you have but I just can't do a picture" - and much of the time people don't bother.

"I have to carefully pick and choose my moment to interact. When am I a celebrity sighting versus when am I going to make someone's freakin' week? Children I don't say no to, for example."