With his latest feature, producer Judd Apatow, the mastermind behind “Knocked Up,” “The 40 Year-Old Virgin” and “Superbad,” takes some calculated risks.

First, a plotline constructed to mock the inexhaustible slew of musician bio-pics finds Apatow outside his filmmaking comfort zone. Second, Apatow’s brand of comedy has become the primary draw, instead of the film’s star, John C. Reilly, a leading man whose most memorable role for Apatow’s audience (or anyone else’s, really) is Cal Naughton, Jr. in “Talladega Nights.” Third, as his eighth production in four years, Apatow is dancing with the devil of overexposure. The fickle 16-to-25-year-old male demographic that has long buttered his bread could turn on him at any moment, and any slip-up could be fatal.

Fortunately for Apatow and the rest of that populist-comedy cabal known as the “Frat Pack” (including Will Ferrell, Ben Stiller, and Vince Vaughn), distinguishing successes from failures can be difficult. One may argue “Anchorman” is a funnier, more original product than the sloppy, drunk-dial of a film that is “Talladega Nights,” but both made boatloads of money. Both spawned their own catch-phrases to be quoted ad nauseum by insipid snots that haunt the arcade in the lobby of your local theater. So the quality of Apatow’s work only means so much to its intended audience.

But is the film any good? That’s trickier.

“Walk Hard: The Dewey Cox Story” plays exactly what you’d expect: another ironically hyperbolic tall-tale comedy that happens to poke fun at other movies…just without Will Ferrell. Reilly can field jokes as Dewey Cox, the loveable dolt who happens to be a musical genius, and his demeanor of perpetual understatement allows for the faith-in-the-underdog mentality that underlies the Johnny Cash biopic “Walk the Line.” But it’s difficult not to long for Ferrell. His comedic presence has, in the past, literally turned a funeral into a riot, and he belongs in a movie like this. His classic Saturday Night Live impersonation of Neil Diamond practically groomed him to play the eponymous role.

Apatow and director Jake Kasdan (“Orange County”) seem to anticipate this absence, slathering “Walk Hard” with cameos big and small—from “Superbad” star Jonah Hill, who plays the grown-up ghost of Cox’s dead brother, to Jack Black, Paul Rudd, Jason Schwartzman, and Justin Long, who literally play The Beatles. Instead of the giddy surprise viewers experience during well-placed cameos, such as Ferrell’s uncredited appearance at the end of “Wedding Crashers,” there is the distinct impression that all of the celebrity drop-ins are mere smoke and mirrors to distract from a lead who, while passably funny, doesn’t have the comedic chops of his compatriots.

As for the humor, Apatow and friends take shot after ever-cheaper shot at the Cash mystique. When the jokes hit, they’re usually propelled by the shock of sheer absurdity: Cox manages to accidentally exterminate his entire family, and spits jive and sings “Love Your Negro Man” when asked to play at an R&B club. When the jokes miss, they never miss big. Consequently, the movie is consistently dumb, though not consistently funny.

“Walk Hard” gives the biopic genre a thorough lashing. Cox fathers dozens of children whose names he cannot remember, experiments with every drug imaginable, and is imprisoned twice, each time following a stint in rehab. Ironically, not unlike the recent Bob Dylan quasi-biopic “I’m Not There,” serious music fans will draw the most from “Walk Hard.” Reilly goes through creative phases that resemble (either implicitly or explicitly) the careers of Johnny Cash, Ray Charles, Elvis Presley, Sonny Bono, Brian Wilson, and, yes, Dylan himself.

While “Walk Hard” demonstrates that Apatow and his cohorts cannot promote from within their own ranks without resorting to their basest kind of humor, fans of “Anchorman” and “Superbad” will be coming out in droves to see it. Whether they will leave the theater satisfied is less certain.