The first thing that surprises you about Don Jon is how thematic it is, and not the way you expect. There's a strong premise here that's mostly hidden until Jon's (Joseph Gordon Levitt) relationship to Esther (Julianne Moore) takes flight, when the young hero has to learn the truth about what he's been both pursuing and avoiding for the whole movie.

Even without the bells and whistles of the script, visuals and characters, Don Jon sets up and pays off the premise in a way that's quite clever and sweet. When the movie's over, you might think it's been a little bit buried under the cinematic approach writer/director/star Levitt takes, but in reality the entire movie is kind of light and fluffy, so the resolution doesn't have to go anywhere near as dark as it could have.

Here's an example. A motif visited several times is the weekly visits to church by Jon and his devout but profane Italian American family. Hi regular confession is a mere transaction of sin and penance – as Jon tells the priest he's masturbated to porn upwards of thirty times that week, he says his Hail Mary's (etc) and goes on his way. It's hard not to wonder whether Levitt is commenting on how institutional religion accepts blind ritual in place of real humility, and it's not the only throughline to be found in the plot.

What Don Jon – surprisingly – isn't really about is pornography addiction. It's a comedy and the original name (Don Jon's Addiction) makes you wonder if there was a much darker tale underneath.

Instead, a lot of what goes on is very funny. The endlessly cocky Jon is a vessel of types. He's violently aggressive behind the wheel, he visits his mother (Glenne Headley), hilarious father (Tony Danza) and sister (Brie Larson – whose character makes you wonder if there isn't a comment on a different kind of addiction as she constantly – and I mean constantly – types on her phone) for dinner once a week in his white undershirt with the football playing in the other room.

And every night, he takes home a different hottie from the cool New York bar where he works. But – as Jon explains during the protracted opening that sets up his character through quick cuts and voiceover – no sexual experience with a real woman can compete with internet porn. Real chicks, he says, just don't do that kind of stuff.

Jon's world is turned upside down when he meets Barbara (Scarlett Johansson). Seemingly the perfect woman, Barbara is beautiful, sexy, feminine, and his parents love her. Jon is smitten, but nothing still comes close to the sexual thrill of his beloved porn. Barbara, disgusted, finds out and makes him promise to swear off it, the sharp end of a much bigger problem in their relationship that soon becomes apparent.

Meanwhile Jon finds himself gradually drawn to the sort of woman he'd never look at twice. He shares a night class (one Barbara has withheld sex to convince him to take) with Esther, nothing like the girl he's usually interested in. Messed up and dressed down, Esther starts to make Jon feel like she understands him in a way Barbara can't accept, especially when she sees him watching porn on his phone and doesn't freak out like his highly-strung girlfriend.

Everyone involved throws everything they have into the characters because Levitt has written (and directs) them as larger-than life people. Jon is king of his own personal fiefdom containing his friends, sexual conquests, car and apartment. The slightly trash-talking Barbara is all hoop earrings and Italian/American drawl (which the couldn't-be-more-waspish Johansson does a great job with). Only Moore's Esther is downplayed, a cool, hippie-like spirit who's ready to accept anything even while she fights her own battle inside.

Levitt directs the rest of the film like his psychologically-oversized characters. The camera moves like the young, horny hero, wheeling, tracking and lovingly caressing the distinctive angular lines of Levitt's face and the New Jersey environs as well as the curves of his partners.

It's hard to know how serious the film wants you to take it because so much of it plays like straight comedy and contains more laugh-out-loud moments than many supposedly funny movies. If this was once a darker project but Levitt (or his producers) decided to make it more likeable, he's done a great job recasting the story, and even if some of it feels more lightweight than you expected, it signals the arrival of a very competent and energetic director.