"The book focused on slow zombies," Pitt, 49, tellsEntertainment Weekly
about the movie based on a bestselling novel by Max Brooks. "We chose to be more dynamic in that we wanted to base all of this on science," he explains. "So it's 'What if we had them move like ants? Or a swarm of bees? Or birds or a school of fish that’s being chased?' One of the first [questions] we asked was how to portray the zombies and how to do it differently because it's been done so many times and been done pretty damn well."