Down, dirty, funny

“Zack and Miri Make a Porno” goes roughly the direction you’d expect it to go. Zack and Miri make a porno, for example. Then the two late-20s roommates, played by the supernaturally easygoing Seth Rogen and Elizabeth Banks, discover that having sex on camera in order to pay the rent will change the temperature of whatever room they’re in together. Permanently.

The film, the latest from writer-director Kevin Smith, pushes its R rating pretty hard, though as with most Smith characters this side of Silent Bob, there’s a lot more raunch in the talk -- the sheer, voluminous, often hilarious verbosity -- than in the action.

Smith will, unfortunately, be going to hell for the inclusion of one outlandishly grotesque sight gag, one the movie (any movie) would’ve been better off without.

Then again, the filmmaker’s entire career has mined the fun, the possibilities and the risks of going too far.

His is an arrested-adolescent sensibility, like that of many who have come along in his wake, notably Judd Apatow and his various guy-centric associates. But Smith can write and, despite his subject and the free-floating, genial skeeziness, “Zack and Miri” has a bright, chipper look to it, thanks to cinematographer Dave Klein, a frequent Smith colleague. Wintertime in Pittsburgh never looked so good.

When Smith wrote the script with Rogen in mind, Rogen wasn’t yet a movie star. Now, coming off “Knocked Up,” “Superbad” and “Pineapple Express,” the actor is courting overexposure.

Yet I don’t get some folks’ weariness or resistance regarding Rogen. Like Michael Cera, he’s a master of making it look easy. It takes skill to come off like you’re making it up as you go.

Example: At one point in “Zack and Miri,” the longtime pals attend their 10th annual high school reunion, where Zack runs into an old classmate (rumble-voiced Justin Long).

Within a minute or so, Zack learns that his schoolmate is making a fine living in adult entertainments featuring all-male casts. “What, you mean like ‘Glengarry Glen Ross’?” Rogen asks.

The way he handles the two seconds prior to his next line is so deft, you think, ahhh, timing. You can teach a performer a lot, but the correct pause before a deadpan rejoinder is not one of them.

Banks is currently on screens as Laura Bush in “W.,” and while she makes full dramatic sense in that context, she’s on top of her game here.

Her slightly generic good looks are made interesting by her refusal to coast on them, and while I wouldn’t call the role of cheery, foul-mouthed Miri nuanced, Banks and Rogen click together in a way that Rogen and Katherine Heigl in “Knocked Up” did not.

Why do Zack and Miri make the porno? They need the money, and the recent economic belly-flop makes that seem like a plenty viable reason.

They shoot their hard-core version of “Star Wars” at the Starbucksian coffee shop where Zack works with his colleague Delaney, played by Craig Robinson.

Their makeshift porno cast is a mixture of game amateurs and low-budget professionals, one of whom is played by Traci Lords (whose authenticity doesn’t really add to the fun).

Jason Mewes, a Smith alum from way back, plays Lester, and his nude scene in “Zack and Miri” arrives very late in a slightly attenuated comedy.

Between this and “Forgetting Sarah Marshall,” I wonder: What is it about seeing a guy’s junk in the final minute or so of a movie that seems ill-considered, somehow?

I’ll try to get back to you on that one.

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‘Zack and Miri Make a Porno’

MPAA rating: R for strong, crude sexual content, including dialogue, graphic nudity and pervasive language.

Running time: 1 hour, 41 minutes

Playing: In general release