C

The trajectory taken by Zack and Miri (Seth Rogen and Elizabeth Banks) in Kevin Smith’s movie is really nothing more than the “Let’s put on a show!” scenario that goes back to MGM pictures of the thirties and forties. But predictably the gleefully vulgar Smith navigates the plot into territory where Louis B. Mayer would never have sent Judy Garland and Mickey Rooney, though the destination opts for not only predictability but sweetness, too.

The title pretty much tells the story: long-time chums and roommates Zack and Miri rent a dumpy apartment in a run-down Pittsburgh house, but still can’t make ends meet. When the duo go to their high school reunion and Miri’s attempt to come on to super-stud Bobby Long (Brandon Routh), the jock she had a crush on, reveals that the fellow’s not only gay but partnered with porn star Brandon (Justin Long), it gives Zack the idea that they too can profit by making a porn flick of their own. (They’ve already achieved a certain YouTube notoriety after some kids posted a shot of Miri in her undies and Zack’s bare bottom.)

Zack’s initial plan is a takeoff called “Star Whores,” but when that plan falls apart, he simply commandeers the cruddy coffee shop where he works—a cousin of the joints in the “Clerks” pictures—for ad hoc night shoots. Having enticed his fellow worker Delaney (Craig Robinson) to pony up the budget and assembled a willing cast—Lester (Jason Mewes), a dim-bulb guy with one enormous talent; exotic dancer Bubbles (Tracy Lords), with talents of her own; Barry (Ricky McCabe), a boy-next-door type; and Stacey (Katie Morgan), the ingenue—as well as cameraman/director Deacon (Jeff Anderson), Zack starts filming. But the prospect of him and Miri getting it on before the lens inevitably leads to the revelation that their feelings toward one another go way beyond the platonic.

There’s some funny stuff here. The opening of the movie has a grubby slacker mentality that recalls “Chasing Amy,” still Smith’s best by a long shot, and it captures the wintry ambience of Pennsylvania in January so well that you almost feel the chill. The high school reunion sequence with Routh and Long is surprisingly deft. And an elaborate final credits postscript summing up what happens after fadeout is a clever bit. Unhappily what intervenes between the first twenty minutes and the last five is pretty puerile stuff. The writing is mostly at the same drearily juvenile level as his previous stuff. That’s partially because Smith doesn’t seem able to pen a single line of dialogue without at least one obscene word or phrase in it. But mostly it’s because he strives for a tone that’s continuously rude and frequently wildly gross. In the final analysis, though, the movie feels self-consciously naughty rather than truly edgy, an adolescent exercise that’s not only behind the curve in terms of basic plot (the little-seen Jeff Bridges movie “The Amateurs” got there first) but never finds a way to make the tastelessness more than mildly amusing. And the overlay of sweetness added to the mix gives the picture a John Hughes feel.

And things are made worse by Smith’s characteristically lackadaisical, laissez-faire style. Though the picture shows real technical improvement over his previous ones—Dave Klein’s cinematography actually demonstrates a rudimentary knowledge of composition—most of the sequences are clumsily staged and poorly paced (Smith himself edited). The most obvious example is the long, dragged-out scene in which Zack and Miri finally make their joint appearance before the camera, behaving so carefully that the result couldn’t titillate the most oversexed observer. It comes across like a cruelly overextended improvisation.

That doesn’t bode well for the actors. Rogen does his usual shtick, and Banks is an attractive presence, but their characters are never really developed, and so after awhile they just cease to be of much interest. Robinson gets laughs with very obvious material involving his extreme racial sensitivity and his love of serving as a porno casting director, but (as on “The Office”) he’s very stiff; and Mewes, Anderson, Lords, Morgan and Mabe do what’s demanded of them without adding much to the printed script. Surprisingly, the most unforced (and actually rather charming) turn comes from Routh, who adeptly sends up his he-man persona; and he’s nicely abetted by Long’s flaming, go-for-broke ostentation.

Technically “Zack and Miri” may represent a considerable advance for Smith, but in terms of storytelling it’s the same old mix he’s always offered. It’s the work of a guy who refuses to grow up, and unfortunately he’s no Peter Pan.