Jeff Kinney had a clear template when it came time to adapt his wildly successful "Diary of a Wimpy Kid" children's books to the big screen.
"I went right to 'A Christmas Story,' says the author, citing the 1983 film based on the stories of radio humorist Jean Shepherd.
"In most kids' movies, the stakes are very high," says Kinney, 39, in Philadelphia last week to promote the movie, which opens Friday. "The world is going to end or somebody is going to die or something awful is going to happen unless the characters do such and such.
"In this movie the stakes are incredibly low. There are two friends who break up and you want them to become friends again. In 'A Christmas Story,' the stakes were perhaps even lower. A kid wants a BB gun.
"We kept reminding ourselves when we were working on the film that you can tell a good story even on the big screen with really low stakes as long as the emotional part of it works."
The result is a movie of rare sweetness and innocence, one that captures the universal agony of adolescence.
On the page, the middle-school adventures of Greg and his best friend Rowley take the form of a journal illustrated with primitive, age-appropriate drawings. The books have resonated big-time with juvenile readers, with more than 30 million copies of the four "Wimpy Kid" titles in print.
Kinney was adamant on a faithful transition to live action.
"I wanted to make sure the voice of Greg, his character and sense of humor, translated really well to the screen," he says. "You want to make sure the movie shares the same DNA as the stick figures in the books."
Casting was key. And "Diary of a Wimpy Kid" hit pay dirt with Zachary Gordon and Robert Capron as, respectively, Greg and Rowley.
Capron, 11, declares that the film is an accurate depiction of his educational experience in Rhode Island, particularly the range of developmental stages that emerges as kids approach their teens.
"It reminded me a lot of real middle school because there are these kids that are 4 feet tall and then there are these kids who are 5 1/2 feet tall and some of them have mustaches and stuff, which is weird."
Gordon is far more worldly, having recently turned 12.
"I read the ("Wimpy Kid") books and loved them," he says. "I actually told my mom I wanted to make a movie about it. But Fox beat me to it." He had to settle for playing the protagonist.
While the movie follows the plot of the first "Wimpy Kid" installment (Kinney is working on the fifth), it does pump up the action.
In the book, Greg is in trouble because he chases some kids with a worm on a stick, the author says. "In the movie Greg is throwing kids into a muddy pit at a construction site in a rainstorm — because it's much more dramatic and cinematic."
Hollywood convention also dictated reworking the final chapter.
"At the end of the book, Greg reverts to being a bit of a jerk. I did that because that's who he is and I was righting the ship," Kinney says. "You can't do that to a movie audience. You can't have them walk out with a bad feeling. So you have to make changes based on emotion."