The filming of "Harry Potter and the Half-Blood Prince" began on a very different note than its five predecessors.
For one thing, shooting on the film began around the same time British author J.K. Rowling published the seventh and final installment of the series, "The Deathly Hallows," in July 2007.
That meant the filmmakers no longer had to guess where the story was headed.
As "Prince" producer David Barron notes, Rowling wrote three of the books after the film series began in 2001 and was always tight-lipped -- even to them -- about what was going to happen.
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However, Jo, as she is known by friends, would steer them in the right direction when they were about to make a misstep.
"I wouldn't do that," she told the filmmakers when they were planning to cut a character out of "The Order of the Phoenix," the fifth in the series.
The author, who also gave birth to two of her three children while writing the books (in 2003 and 2005), was also freed up to visit the set more.
"I think now that she has finished the books and there's some separation, it's a little bit easier for her to come to the sets," said David Heyman, who has been a producer on all of the Potter movies.
But even as Rowling has become more of a presence, the filmmakers have nothing but praise for her. "She always has made herself available -- even at her busiest times -- and she never tries to railroad us," Barron said.
Chris Columbus directed the first two Potter movies before turning over the reins to Alfonso Cuarón for the third and Mike Newell for the fourth. All of them turned down chances to direct more. David Yates took over for the fifth film, directed "Prince" and is working on "Hallows."
"I love to work," said Yates, who directed the BBC version of "State of Play." "I love being industrious. It's a privilege."
Though having the same director for the last four films has given the production a certain sense of continuity, one of Yates' goals is to give each film a different look. For "Prince," there is a shimmering quality to the lighting.
"Prince" was set to come out late last year, but after the writers' strike, Warner Bros. needed a summer blockbuster, so it held up the release. Yates said that the delay gave him a little more time to "fiddle" with the film but that it is essentially the same because he had to start working on "Hallows."
Certainly, as all the filmmakers note, the Potter franchise was fortunate in casting Radcliffe, Watson and Grint for the three main characters nearly a decade ago.
"We couldn't predict how good they would become both as actors and individuals," Heyman said. "All three are blessed with decency and humility. We're very lucky."
Added Barron, "That all three of them have developed into really powerful actors is an amazing bit of luck or foresight. It's hard to know which."
Though characters come and go and grow, there are constants about making a Potter film: You have to stay faithful to Rowling's vision and the fans' expectations and still make a manageable film.
The prospect of condensing "Hallows" was so challenging that the filmmakers decided to turn it into two movies, an idea Rowling supported. Still, there will be cuts in "Hallows," as there were in "Prince."
So far, the fans seem accepting.
"At the London premiere ... it was pouring down rain and hailing," Heyman said. "But there were more fans than I've ever seen."