The Hogwarts gang is growing up as "Harry Potter and the Prisoner of Azkaban" (search) prepares to cast a spell on the box office this weekend.

The third film in the "Potter" series, which opened nationwide Friday, reflects Harry and friends' entry into the confusing world of adolescence with a darker, more mysterious tone.

"A big theme of this film is time, in the sense of what you leave behind and what you travel to," said the film's director, Alfonso Cuarón (search).

The character's complex world is marked by some notable factors on and off screen. For one, the actors playing Harry, Hermione and Ron are growing into teens before fans' eyes. There's also a new face in the headmaster's office: Michael Gambon (search) steps into Professor Dumbledore's shoes in place of Richard Harris who passed away last year. And behind the camera, Cuarón is at the helm instead of Chris Columbus, who directed the first two films.

Cuarón, who made the very sexually-charged 2001 coming-of-age film "Y Tu Mama Tambien," has quite a different style than Columbus, known for family films such as "Home Alone" and "Mrs. Doubtfire."

J.K. Rowling, who wrote the wildly popular series of books the movie are based on, told Fox News she was thrilled with Cuarón's take on her work.

"Alfonso has very good intuition about what would and wouldn't work," she said. "He's put things in the film that — without knowing it — foreshadow things that are going to happen in the two final books. So I really got goosebumps when I saw those two things."

The movie has already gotten a thumbs-up from the press.

"Right from the start, 'Azkaban' seemed so much better than the first film, 'Harry Potter and the Sorcerer's Stone,'" wrote Foxnews.com columnist Roger Friedman. "It's actually a little edgy and a whole lot sharper. Cuarón has taken the characters and the story as we are painfully aware of it and breathed new life into it."

However, Cuarón's change of tone was risky given the passion with which Potter fans greeted the first two films.

Steven Kloves, the screenwriter who has adapted all the "Harry Potter" movies, said the director had to balance his own vision with the already-established world of the series.

"Alfonso will tell you that he inherited a glorious world, an incredible paint box to play with. And what he did was, he chose his own colors," Kloves said. "I think the world is recognizable, it's just been given a bit of a darker cast, which reflects the story and what Harry is going through emotionally."

In "Azkaban," Harry, who is being hunted by a renegade wizard, is more cynical and wiser after facing down his nemesis, Voldemort, in the previous movies. Harry, an orphan, is also slightly bitter and lonely — and tired of dealing with bad guys and monsters and school bullies.

This time the characters wear modern street clothes for much of the movie, instead of the school-uniform robes seen so often in the previous films. And the school looks more worn and earthy — as if they'd been battered for hundreds of years by misplaced adolescent magic.

Actor Daniel Radcliffe, now 14, said Cuarón didn't shy away from the bleak touches that showed magic has a dangerous side.

For instance, Potter fans are familiar with the Whomping Willow, the limb-swinging living tree that battered a flying car in the second movie — but are they prepared to see it whack sweet little bluebirds into puffs of falling feathers?

"There's slightly more twisted humor in it, which I always love — I've got quite a black sense of humor," Radcliffe said.

In real life, there is something of a dark cloud looming over Radcliffe, Emma Watson (Hermione) and Rupert Grint (Ron). The actors are aging faster than their on-screen counterparts, fueling rumors that they may not return for all the Potter films.

They are already in the midst of filming the fourth movie, but as they go on to do the fifth and sixth films, the actors will likely out-pace the characters and may have to be replaced.

Radcliffe, who was plucked from relative obscurity to play Potter, said he knows he may not be able to star in the full series, but isn't pleased with the idea.

"If it happens, it happens I suppose," Radcliffe told Fox News. "I'm not going to lie and say that I would be absolutely fine with watching someone else play the part, you know, it would be weird."

Fox News' Mike Waco and the Associated Press contributed to this report.