The purpose of a hidden camera movie is simple: expose the way people treat each other when they think they’re in private.

Movies like Sacha Baron Cohen’s “Borat” or Johnny Knoxville’s new “Jackass Presents: Bad Grandpa” (Rated R; 92 Min.) do not have the same narrative restrictions classically- scripted movies do. They are tied together using minimal plot that drives the movie forward when the camera isn’t focused on an awkward situation. The whole point is to give an insight into the way Americans think and respond when confronted with an uncomfortable moment.

“Bad Grandpa”’s plot can be summed up in one sentence: Irving Zisman (Johnny Knoxville) has been saddled with getting his grandson across the country to the kid’s deadbeat father. Knoxville and director Jeff Tremaine choose to take the character right through the heartland, from Kansas City to St. Louis and all the way to North Carolina.

Where “Bad Grandpa” deviates from “Borat” is the type of people confronted. Knoxville rarely makes

the situations personal; Cohen as “Borat” spends time with specific individuals to show audiences racism and other troubling issues still aflame in parts of the country.

Instead, Knoxville is content just eliciting weird looks and laughs out of his victims. At first it is amusing to see people along the street exposed to the weird things Zisman does — there is a great gag before the title reveal that sets up the movie as something more provocative. But once the movie settles into its loose storyline Knoxville becomes way too comfortable with the responses he gets. Instead of using his persona to reveal how people treat senior citizens he just makes some poop jokes.

Only once or twice does “Bad Grandpa” really surprise. Otherwise, Knoxville just plays it safe. Watching innocent civilians’ responses, typically laughter, gets old pretty quick. Knoxville rarely tries to engage bystanders to reveal their true nature, and the one time he makes a pass at a subculture — “Toddlers and Tiaras”-style beauty pageants — was already revealed in the trailer.

Maybe it’s not right to expect more from the guys behind “Jackass.” After all, they were responsible for shooting a porta-potty up in the air with one of their cast inside. Poop is their game.

But what is the point of at least piecing together a narrative if you don’t have something you want to say? “Jackass” would have been fine with a fourth installment of their prank movies. In fact, it would have made plenty of money. Creating characters tells the audience you want them to make a genuine connection with the movie, but Zisman never seems like anything more than Knoxville in old-age makeup. His personality isn’t different enough.

Despite the relatively safe nature of the movie, Knoxville has several good gags up his sleeve that will keep his target audience, and maybe even a few older folks, entertained. The interlude scenes between Zisman and his grandson establish somewhat of an interesting relationship, but even the emotional scenes are played for laughs.

Even though “Bad Grandpa” doesn’t provide any solid breakthroughs for the hidden-camera genre, it is funny enough to keep the audience interested. But a little more differentiation between a movie with a plot and a practical joke flick could have really made it stand out.

Karsten Burgstahler can be reached at [email protected] or 536-3311 ext. 261.