This year marks the 15th anniversary of the very first Harry Potter movie, 2001's Harry Potter and the Sorcerer's Stone, which started the lucrative franchise and introduced the world to three little-known actors who would go on to become global superstars, Daniel Radcliffe (Harry Potter), Emma Watson (Hermione Granger) and Rupert Grint (Ron Weasley). Like many big-budget blockbusters based on a wildly popular novel series, the casting has to be just right to ensure any kind of success, and it's safe to say that the filmmakers behind Harry Potter and the Sorcerer's Stone certainly got the casting right. Today we have new details from casting director Janet Hirshenson, who reveals the casting process wasn't easy by any means, along with the unusual yet perfect reason why Daniel Radcliffe was cast in the role of a lifetime as Harry Potter.

The Huffington Post caught up with Janet Hirshenson to delve deeper into this historic film that kickstarted a lucrative franchise. What's interesting is that Daniel Radcliffe had originally taken himself out of the running to play Harry Potter, because he wanted to quit acting after starring as a younger version of the title character in the 1999 mini-series David Copperfield. Producer David Heyman, who knew Daniel Radcliffe's father, a talent agent, ran into both father and son during a night at the theater, and convinced young Daniel to audition. Here's what Janet Hirshenson revealed about why Daniel Radcliffe beat out an unspecified young actor, who was the only other top choice for the role.

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"When we sat down to look at the tests. There was another guy that we kind of liked, too. There were two. (Director) Chris Columbus right away liked Daniel, but there were a couple of people who went, 'Hmm. That other kid's interesting.' So we thought, 'Let's all go to sleep on it. We'll come back and look at this again.' We went back and looked at Daniel again. The other kid was terrific and very vulnerable and very Harry-looking, but besides that, Harry was going to become a very powerful kid, too. And Daniel had both sides. He was very vulnerable, but the other kid ― it was like, he [was] not going to have the balls that Daniel has, to put it that way."

The casting director also reveals that there was "a push for the actor who did Billy Elliot," Jamie Bell, who made his feature film debut as the title character in that 2000 film. However, he was 14 years old at the time, which was just too old since one of the casting edicts was that they cast actors who were the actual ages of the characters they're playing. Once the filmmakers finally landed on Daniel Radcliffe, Emma Watson and Rupert Grint for Harry, Hermione and Ron, Janet Hirshenson revealed what happened after all three actors were told they had won their respective roles.

"After we chose, they pulled the three of them up to Chris's office, not telling them they got the part, but they were standing there, the three of them, looking at each other, probably figuring, 'I think we may be it.' So they told them they had it. 'Yay, yay, yay,' and then they started chattering amongst themselves because they hadn't known each other very much. Emma asked Daniel if he liked the books and he said, 'Yeah, I like WWF better, and she did a harumph or some Hermione thing that was so perfect, just as herself. '[Gasp] WWF!' And we just all were like, 'Whoa, this is them.'"

Another reason Daniel Radcliffe won the role is that he had the right color eyes, with Janet Hirshenson revealing that a number of young actors were quite good, but they were never strongly considered because they didn't have either blue or green eyes, to match Harry Potter's blue-green eyes from the books. Jamie Bell had brown eyes, which is one of the reasons he was eliminated from contention, along with his age. One of the reasons why it was so hard for the filmmakers to cast Harry Potter, and all of the other characters for that matter, is the producers had a strict rule of hiring only British actors to play these characters. In fact, the late comedic legend Robin Williams wanted to play Hagrid so badly he called director Chris Columbus to ask for the role, which ended up going to author J.K. Rowling's first choice for the role, Robbie Coltrane. Here's what the casting director had to say about Robin Williams asking for the Hagrid role.

"Robin Williams had called [director Chris Columbus] because he really wanted to be in the movie, but it was a British-only edict, and once he said no to Robin, he wasn't going to say yes to anybody else, that's for sure. It couldn't be."

Harry Potter and the Sorcerer's Stone opened in November 2001, taking in $317.5 million domestic and $974.8 million worldwide, figures which are the second best in the franchise behind only the final movie, Harry Potter and the Deathly Hallows - Part 2 ($381 million domestic, $1.3 billion worldwide). The franchise continued this year with the spin-off Fantastic Beasts and Where to Find Them, which is set 70 years before Harry, Hermione and Ron first set foot in Hogwarts. The franchise also continued on the London West End stage with the new play Harry Potter and the Cursed Child, which will debut on Broadway in 2018.