Radcliffe, who is now 19, was 11 when he was cast as the boy wizard for the series' 2001 debut. Watson, now 19, was 10 when she auditioned for the whip-smart Hermione Granger. Grint, the eldest of the trio, is 20.
"I've probably been Ron as long as I've been Rupert," says Grint, who plays Ron Weasley, the ginger-haired, perpetually hungry friend of Harry and Hermione.
The cast and crew have taken a break from filming Rowling's last "Potter" book
- to be spread out in two films - to publicize the series' sixth installment, "Harry Potter and the Half-Blood Prince," which arrives in theaters Wednesday.
Early reviews of the movie, the second one directed by David Yates, have been positive; both Variety and The Associated Press suggested it was the best "Potter" film yet. The movies have become progressively more complex, darker and realistic
- even amid the fantasy world of Hogwarts School of Witchcraft and Wizardry.
As the films have matured, so has the cast.
More so than any other installment, "The Half-Blood Prince," shows that Radcliffe, Watson and Grint have gone from children to young adults. With the end of the series and a sense of graduation looming, its young stars appear to have emerged from the most treacherous of adventures
- child actor stardom - as remarkably grounded people and increasingly talented actors.
To watch the first "Potter" film is to be reminded how young the actors were when they began.
"For me to look back on the old films is an almost entirely destructive thing to do," Radcliffe says. "I just torture myself over it. I mean, I was young. I can't be held accountable for the performance I gave in the first two films: I was 11 and 12. I wasn't like Dakota Fanning ... who could seemingly just do it. It was very much a child's performance."
Such awareness is common for Radcliffe, who goes by "Dan." Shy as a child, he has grown into a quick-witted, animated 19-year-old who relishes frantic chatter about indie music, the behind-the-scenes aspects of filmmaking and his burgeoning love of acting. Michael Gambon, the award-winning British stage and screen actor who plays Hogwarts headmaster Dumbledore, says, "He's not a boy anymore. ... You can see it in his face."
The many lauded Brit actors of the "Potter" films have influenced Radcliffe
- perhaps none more than Gary Oldman, who played Sirius Black in several of the films, most notably the third, 2004's "Harry Potter and the Prisoner of Azkaban." Interestingly, Radcliffe pegs that film as the moment he realized he loved acting.
"Something happened at the age of 14," he says. "I started taking it more seriously, which meant I started having more fun."
He says his parents (who waited patiently in a room next door during the interview) always reminded him that he was "not obliged to just carry on doing this." But Radcliffe grew more confident and began considering his active imagination, which he attributes to being an only child, as his greatest asset as an actor.
"I would have always wound up in the film industry somehow, probably as an assistant director or something like that. It just so happened that it turned out this way," he says. "I want to be somebody who works with the crew rather than for himself."
Since then, his progress has been apparent with each new "Potter" film
- "a biannual review," Radcliffe calls it. He has begun moving away from Harry Potter, including a hilarious cameo in Ricky Gervais' TV series "Extras," and a well-reviewed performance in a revival of Peter Shaffer's "Equus," which ran in London in 2007 and on Broadway in 2008. Radcliffe played a deranged stable boy who completely disrobes
- a scene much written about media.
Radcliffe counts his last year as both his "biggest leap" and an "overwhelming blitzkrieg of camera flashes."
The soft-spoken Yates - who is directing the final two films, to be released by Warner Bros. in November 2010 and summer 2011
- is credited with helping the young cast mature.
"They're getting more experience outside of the film set and they're bringing that to the floor," he says. "People are acknowledging it for `Half-Blood Prince'
- but you haven't seen anything yet."
Watson has a hard time recalling the beginning.
"This all happened to me so young," she says. "It's very hard to go back to that time and be like, `Did I want to do this?' It feels very foggy."