ORLANDO, Fla. -- If you want to make a film about a not-so-modern Stone Age family, it helps to start out with a wish list.
"If I was going to see a caveman film, what would I want to see?" asks Kirk De Micco, co-director of The Croods.
"We've never seen computer-generated cavemen," says Chris Sanders, the other co-director. "We wanted to do for cavemen what 28 Days Later did for zombies. Our cavemen are very gifted, physically. They can run 40-60 miles per hour, throw rocks huge distances and hit their targets. But mentally, they have beginners' minds.
"Tar is way up that list," says De Micco. "But nobody had ever animated it. We had to manage a tar pit, and the animation that we used was a simulating of flowing cloth."
They wanted their Stone Age to have a volcano.
"We have a pyroclastic flow in the film that I've never seen the likes of before," Sanders says. "A titanic volcanic collapse that is the final curtain on this world as they knew it."
De Micco adds that "there are no man-made things in the movie," which opens Friday. "It's all nature, all exteriors, and that's a huge undertaking. Just caves and cavemen and forests."
They based the human characters, voiced by the likes of Oscar winners Nicolas Cage and Cloris Leachman, as well as Emma Stone and Ryan Reynolds, on assorted members of the animal kingdom.
Of Eep, the cave-teenager voiced by Stone, Sanders says, "Her movements are based on a cat's.
"Grug [her dad, voiced by Cage] has movements based on a gorilla. Gran's [Leachman] movements are based on a crocodile, the way they walk. Fanny, the youngest, has movements based on a terrier."
The directors knew their movie would be compared to the lucrative "Ice Age" series. The Hollywood Reporter says " The Croods is too uneven to help it approach that series' mammoth market share. But its mostly fast-moving roller coaster of kinetic action and its menagerie of fantastic creatures -- from cute to menacing -- should keep kids entertained."