The British beauty reveals her mum and dad worried when they saw how much the then nine-year-old Watson threw herself into rehearsing for the role of Potter's pal Hermione Granger ahead of her screen test.
And the 20 year old admits she would have been devastated if her parents had banned her from starring in the franchise.
She tells Marie Claire magazine, ""I started working on the audition at nine in the morning and didn't stop until five in the evening. The tape was just me doing the same thing hundreds of times over, until I got it exactly right. I was just amazed at my stamina.
""The grown-ups said, 'We had to stop you; you wanted to keep going.' I've always been like that; I give 100 per cent. I can't do it any other way.
""I would never have forgiven my parents if they had made me turn down that opportunity. I just loved performing. It just made me feel alive. It's scary, but that's part of it. I think it's important to have that extra adrenaline. It gives you that extra zing.""