Knoxville Hams It Up with 'Little Jerk' Jackson

Johnny Knoxville spins off his outrageous "Irving Zisman" character for a whole new Jackass experience, Bad Grandpa, and he tells ET that he had to don the 86-year-old grandpa disguise because, "I can't prank the public as myself anymore."

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"People are really sympathetic to an older person, and when you have a cute little angel beside you, they're even more sympathetic, so it was really the perfect premise [to pull pranks]," says Knoxville, adding that despite his diminutive co-star Jackson Nicoll being a "sweet" kid, "We just hired him to be a little jerk, and boy did he exceed all our expectations. ... he's a good kid, he just knows that we like him to act up."

In theaters Friday, Jackass Presents: Bad Grandpa is a cross-country tale captured on hidden cameras that pairs Knoxville with his pint-sized, Fun Size co-star Jackson as his grandson and unlikely companion, 8-year-old Grandson Billy. As Irving introduces the young and impressionable Billy to people, places and wild situations -- male strippers, a child beauty pageant, a funeral home service and a biker bar to name a few -- cameras capture the reactions of real people who just can't believe what they've just seen.