A hard-R version of "Betty White's Off Their Rockers" intermingled with the part-documentary/part-narrative format of 2006's "Borat: Cultural Learnings of America for Make Benefit Glorious Nation of Kazakhstan," "Jackass Presents: Bad Grandpa" puts Johnny Knoxville's (2013's "The Last Stand") elderly, wildly inappropriate alter ego, Irving Zisman, in the lead role of his own feature film. Writer-director Jeff Tremaine (2010's "Jackass 3D") and co-writers Knoxville and Spike Jonze bust through every tasteful barrier they can, but the movie's tone is so good-natured, as unlikely as that may seem, that one cannot help but laugh at the sight of projectile sharting and a penis getting hopelessly lodged in a soda machine. The situations are droll, raunchy and pretty much expected by themselves, but where the picture gets its comic inspiration is in its use of unsuspecting real-life passersby and onlookers. Not realizing they are being filmed "Candid Camera" style, their mortified, uncomfortable, deer-caught-in-the-headlights reactions are frequently sidesplitting. The story happening around these pranks is on wobblier terra firma, but at least Knoxville and the irresistible Jackson Nicoll (2012's "Fun Size") share a winning, anything-goes rapport.

When 86-year-old Irving Zisman's battle-axe of a wife takes a turn for the worst and passes away, he is both upset and overjoyed—with an emphasis on the latter. Her funeral coincides with the appearance of his drug-addicted daughter (Georgina Cates), who informs him he must take his 8-year-old grandson, Billy (Jackson Nicoll), across the country to his estranged father (Greg Harris) while she serves a jail sentence. Irving is, shall we say, exceedingly randy for his age—"I might be too old to stir the gravy, but I can still lick the spoon," he tells one virtually speechless lady on the street—and not exactly the most fitting guardian of an impressionable little boy. Then again, he does grow to genuinely care about him during their cross-country journey, which is more than can be said for Billy's deplorable dad, who sees him strictly as a means of getting a monthly $600 check for child support.

When "Jackass Presents: Bad Grandpa" tries to pluck the heartstrings, it's not particularly convincing. Whether one watches it as fiction or mockumentary, at no point does the viewer buy into Irving and Billy as a legitimate grandfather and grandson. Zisman has been readily seen getting into mischief in the previous reality show-inspired "Jackass" trilogy, and all of the professional make-up in the world cannot hide the knowledge that Johnny Knoxville is behind the wrinkled latex. With that said, Knoxville and Nicoll are awfully sweet together, even when they're bonding over beers and talk of finding tail (it helps that one particular montage is scored to King Harvest's soaring "Dancing in the Moonlight," which 2002's "A Walk to Remember" already proved can make any movie better). There is not only a special, fearless quality to what Nicoll does onscreen, but also a poise and gravitas that transcends the work of many child actors around his age.

As is the usual with films adapted from skit comedy, "Jackass Presents: Bad Grandpa" is hit and miss as it stretches itself out to an hour and a half. This one, however, works more than it doesn't and moves at a clip rate. If one set-piece isn't working, rest assured that a new and probably better one is minutes away. From an insanely unorthodox visit to a bingo game, to a trip to a male strip club's "ladies night," to, best of all, Billy's admittance into the Carolina Cutie Pie Beauty Pageant, where he performs a pole dance to Warrant's "Cherry Pie," "Bad Grandpa" gets its inspiration from the regular folks who have no clue they've just become the unwitting supporting characters of a Paramount Pictures comedy. This is total cinematic junk food, not a fraction as perceptive about its satirical pointedness as "Borat" or 2009's "Brüno," but that doesn't mean it isn't sinfully tasty while it lasts.