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The British actress was just 10-years-old while Tom was 13 when they started shooting the 2001 movie, the first in the film's franchise.
In the upcoming one-off, the 32-year-old star admitted she fell head over heels for Felton, who played Draco Malfoy, during a tutoring session on the first flick and would eagerly look out for his name on the call sheet.
During the special on HBO Max and NOW, titled Return To Hogwarts, Emma recalled: "I walked into the room where we were having tutoring.
"The assignment that had been given was to draw what you thought God looked like, and Tom had drawn a girl with a backward cap on a skateboard. And I just don't how to say it - I just fell in love with him.
"I used to come in every day and look for his number on the call sheet, it was number seven, and if his name was on the call sheet, it was an extra exciting day.
"He was three years above me and so for him he was like, 'you're like my little sister'".
She added that nothing "ever, ever, ever, happened romantically," but they "just love each other".
Unsurprisingly Tom admitted he was aware of Emma's crush on him and instead of acting on it, he became incredibly "protective over her" and their friendship.
The 34-year-old said: "I think I was in the hair and make-up chair, and someone said something along the lines of, 'Yeah, she had a crush on you.'
"I became very protective over her. Yeah, I've always had a soft spot for her and that continues to the day.
"There's always been something that's like, I don't know, a kinship."
The pals have been at the centre of romance rumours for several years. Although there's nothing romantic between the two, they regularly share photos together to their social media.
Meanwhile, Little Women star Emma is dating businessman Leo Robinton while Tom is currently single.
Harry Potter: Return To Hogwarts airs on New Year's Day on HBO Max and NOW in the UK.
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