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.
See what they looked like in the beginning.
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 biennial 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."
Take a look at how "Harry Potter" took over the world.
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."
Watson has acted in a few other films (a voiceover in 2008's "The Tale of Despereaux" and the 2007 BBC film "Ballet Shoes") but she has spent most of her spare time throughout "Potter" - and this is very Hermione-like - studying. This fall, she'll attend Brown University, says producer David Heyman. (Watson isn't discussing her plans publicly.)
"I would have exploded if I hadn't had school to ground me and focus me," Watson says.
She expects to continue acting, but says college felt like the obvious decision.
"The three of us have been working solidly since we were 10 years old," she says. "I just need a little bit of normality for a while, just a little bit of space to work out what I want and who I am - all the usual stuff. It's just something I always wanted to do."
She plans to study literature and art, but she has also shown interest in fashion. She signed to a modeling agency about two years ago.
"Fashion's great because you're able to recreate yourself whenever you want," Watson says. "Dan had time to go away and do `Equus' on Broadway and break out of `Harry Potter' a bit, and I was always studying. So my way of getting casting directors to look at me in a slightly different way was modeling."
Heyman, who has been with the series from the start, said: "I see their individuality really shine through as actors and as people. But at the same time, I see the same kids who are very much still filled with a sense of wonder and still have a sense of humility and don't believe the hype."
The bemused Grint - whom "Azkaban" director Alfonso Cuaron once said was the one most likely to become a star - remains clearly grounded, even if he's used his earnings to purchase a hovercraft. That playfulness is perhaps an essential quality to Grint, who was never inclined to view acting as a job.
"I don't think I ever really made the connection of it being a career," he says. "It was just something that was fun to do. In the early ones, I don't think I took the acting too seriously. I just read the lines and got on with it. Over the years, you start to take it more seriously with different directors coming in."
Grint has starred in two films not yet released: "Cherrybomb," a boozy teen comedy set in Belfast, Ireland; and "Wild Target," a film about a retiring hit man that stars Bill Nighy and Emily Blunt.
He says he's enjoyed the "more adult" roles and feels more comfortable in front of the camera after an awkward adjustment: "This is something I was kind of thrown into," he says.
Teenage years are typically uncomfortable ones - years that few would want stored on celluloid. Grint compares the "Potter" films to "a really expensive home video."
"I guess we've all kind of grown up," he says.
The paychecks have grown in tandem, too. Forbes reports that Radcliffe made $25 million last year. They are all legally adults now and are beginning to live on their own.
It's clearly a strange ride for the trio, who have only a vague sense of how this all began for themselves. Though they don't generally socialize offset, the camaraderie of going through it together has clearly helped.
"To have someone that's in the same boat as you is a relief," Watson says. "I wish in a way that there had been a fourth in the trio that was perhaps a girl, but they've been pretty great."
All three are certain of one thing: When they wrap the last "Harry Potter" scene, there will be tears. Their adolescence is forever intertwined with the movies.
"These are some of the most important years of my life and I won't be able to look back on any frame of this film without it being linked to a dozen memories," Radcliffe says.
By Jake Coyle