“Zack and Miri Make a Porno” is a good movie, easily writer/director Kevin Smith’s best since “Dogma.” You know what would make it great?

If Judd Apatow’s name followed the “directed by” tag.

This isn’t wishful thinking on my part. Kevin Smith has made a movie that really and truly wants to be a Judd Apatow flick.

It follows the same formula. Step one: Irresistible comic premise. In this case, two best friends make a porno to solve their financial woes. Step two: Lace that taboo-busting setup up with genuine warmth. Said best friends realize their true, more romantic feelings towards one another during the shoot. Step three: Cast it with the best from the Apatow stock. In this case, Apatow players Seth Rogen, Elizabeth Banks and Craig Robinson do most of the heavy lifting.

Something’s slightly amiss here, and that something is Smith himself. That’s bothersome, especially when you consider that Kevin Smith laid the groundwork for Judd Apatow’s recent successes.

Take everything from “Clerks” through “Jay and Silent Bob Strike Back,” and you can trace his influence on Apatow -- relatable characters delivering profoundly raunchy dialogue.

Problem is, Smith never matured as a writer. His work post-“Dogma” either backslid (no one, repeat NO ONE, needed a “Clerks 2”) or devolved into maudlin tripe (“Jersey Girl,” anyone?). As a director, “Zack and Miri” is a huge step up -- this flick looks about one billion times more polished and professional than “Clerks” did.

Not that that’s a stretch these days -- I've seen Heath Ledger tributes on You Tube more polished than Smith’s debut.

But his words still ring hollow.

The first 10 minutes of this movie can be difficult to take, and the copious vulgarities have nothing to do with that problem. Rather, it’s the unnaturally mannered cadence of the words. Smith is clearly showing off, but his linguistic stylization rings “cutesy” instead of “clever.”

Only the brilliant exchange between Rogen and a never-better Justin Long shines in this opening stretch and their dialogue feels more riffed than it does scripted. Most people just don’t write down “The Wiz” references.

Again, one of Apatow’s strengths is his use of improvisation -- there’s an unforced honesty to his characters’ speech that is absent at the beginning of Smith’s flick. The dialogue even renders unfunny a scene where Rogen’s character burns himself in a very unfortunate place with a Hot Hands packet. That material should be a slam-dunk, but it falls flat. I don’t hear Zack and Miri speaking; I hear Kevin Smith.

Thankfully, the movie gets much better after that. I don’t know if Smith’s words get better, or maybe the cast gets more comfortable delivering them, but a good 70 to 75 percent of this flick is a pleasure, courtesy of Rogen, Banks and Robinson.

Robinson is having a banner year. He steals “The Office” whenever he’s on (as Darrell), he walked away with “Pineapple Express,” and he does his best work in this as Delaney, the slightly unwilling porno producer. This is the biggest role Robinson’s gotten yet, and he tears into it -- from his wearied rants about his wife to his quietly emerging enthusiasm over the movie, Robinson is a treat.

Banks and Rogen are even better. These guys have worked together off-and-on since “The 40 Year Old Virgin,” and their easy, natural chemistry helps allay the obvious physical discrepancies between the two.

Rogen is, admittedly, covering fairly familiar territory -- the genial slacker forced to grow up -- but he’s so relaxed and funny it doesn’t matter. And, if there’s any justice, Banks should be the next Julia Roberts. She is so funny and sexy and captivating it’s a wonder she’s not a bigger star.

It also helps that this material is very close to Smith’s heart. In a sense, this is the most autobiographical thing he’s ever made, with the lo-tech, high-spirits guerilla porno shoot here not far removed from his own experiences making “Clerks.” That extra feeling translates well, and barring some disgustingly obvious song selections, Smith’s heart-to-fart ratio is pleasantly Apatovian in its proportions.

Yet Smith keeps getting in his own way. He gathers a game supporting cast, from “Clerks” star Jeff Anderson (so good there as Randal) to actual porn stars Traci Lords and Katie Morgan, and he gives them almost nothing to do. They have maybe one funny bit apiece and then spend the rest of the movie watching Rogen, Robinson and Banks get all the good stuff. Comedy is ensemble work and the ensemble here just doesn’t cut it.

Worse is the curiously unfocused narrative.

For all the economic woes Zack and Miri have, plot-wise things are much easier. There’s no real tension, no conflict (other than the will-they-or-won’t-they-setup between the leads) standing in the way of the porno -- these characters hang out, shoot some sex and then the movie ends.

Despite my misgivings, this is a good movie. It’s getting slaughtered in the box office, and it deserves better than that. Normally, here’s where I’d try and come up with something cute to end on; instead, I’ll just urge you to check this one out. Movies are expensive, so catch a matinee. But do give it a shot.

Pretend it’s an Apatow movie, if that helps.