Don Jon 

It’s difficult to rate a movie that doesn’t really start until its last half hour. Joseph Gordon-Levitt’s debut as a writer-director has a solid premise: It’s a romantic comedy told from a not-so-romantic guy’s perspective. The problem: About 70 percent of Don Jon is a one-joke comedy populated by character types lifted straight from “Jersey Shore.” The last 30 percent almost redeems it — but not quite.

Actors writing a role for themselves often go overboard on their character’s depth, intelligence and sensitivity (Zach Braff in Garden State, anyone?). Gordon-Levitt, who’s already known for playing characters of depth, intelligence and sensitivity, has done the exact opposite. The titular Jon is a Jersey bartender who spends his free time pumping iron and hanging at clubs with his buds. His painfully histrionic Italian parents (Tony Danza and Glenne Headly) want him to find a nice girl and settle down. But Jon has never met a woman whose long-term company he preferred to the endless supply of voracious, surgically enhanced sirens waiting for him on porn sites.

Online-porn addiction isn’t some trumped-up movie problem. “Porn is … shaping men’s physical and emotional interest in sex on a very fundamental neurological level,” wrote Davy Rothbart in New York Magazine, one of those who’ve claimed the syndrome is changing the face of relationships for a generation.

As a screenwriter, Gordon-Levitt seems to have taken his cue from such commentaries. In voiceover narration, Jon explains graphically why he prefers virtual sex to the real thing — yes, even when the latter finally happens with the perfect 10 he’s been courting, Barbara Sugarman (Scarlett Johansson).

The joke is that, while Jon’s laptop screen has him in thrall (the chime of a booting Mac gets him hot and bothered), Barbara is no less suckered by the depictions of True Love in PG-13 flicks. She’s Jon’s exact female counterpart, and she’s set on transforming him into her notion of a romantic lead.

It’s a good bit of satire, as is the recurring gag in which Jon goes to confession and gets his sexual sins absolved, then says his Hail Marys between pants at the gym. Once we’ve appreciated those barbs, however, all we have to watch is a courtship between two excruciatingly shallow people. Both actors play their abrasive, Joisey-accented characters with all the subtlety of a “Saturday Night Live” sketch. It’s quite possible to mock an ethnic subculture with tact and love, but Moonstruck this is not.

That’s why it’s a shock to see Julianne Moore pop up, without warning, as an older classmate in Jon’s night course. Though she initially seems aggressively quirky, her character soon starts showing nuances of humor and emotion heretofore missing from the film. Her interactions with Jon transform him into less of a caricature, too, coaxing the movie toward a surprisingly graceful and insightful conclusion.

It’s hard not to wish Gordon-Levitt had compressed the first two thirds of the film into a half hour and taken more time to explore the part where his character evolves. Perhaps he hoped to inveigle real-life Jons into absorbing the movie’s message by luring them with the time-honored techniques of MTV: hot chicks, loud music, broad comedy and grinding. (Let the squeamish be warned: The movie features a cornucopia of porn clips.)

If he succeeds in making the unthoughtful think, good on him. Moviegoers outside Jon’s demographic may find Don Jon’s raunchy candor refreshing in a rom-com context, and its last half hour touching. Way too much of its satire, though, is about as thought provoking as a montage of money shots.