Here’s a question we’ve considered before. When does a film about a misogynist become a misogynistic film? The answer generally concerns the extent to which we are invited to identify with the protagonist.

There is little doubt that Joseph Gordon-Levitt is a tiny bit in love with the monster he has created for his debut feature. Buffed up into a mahogany triangle, the actor offers us a grotesque New Jersey variation on Saturday Night Fever's Tony Manero. He lives for his car, the church, his porn and the girls. Any pesky romantic affections are subsumed in the excitement of sexual conquest.

For all Gordon-Levitt's investment in the title character, however, Don Jon ends up revealing a soft heart and a decent temperament. Levitt is excited by Don when he's a jerk, but he only really likes him when he is trying to be something else.

Unfortunately the film does have other problems. Gordon-Levitt has come up with an interesting, unsettling notion: might a contemporary Don Juan be keener on internet pornography than sex in the real world?

He makes the case effectively, but – as Mozart, Byron and Shaw would surely agree – it’s more interesting to have your hero flee angered husbands across dangerous rooftops than have him stare repeatedly at screens while administering five-fingered shuffles. The arrival of Scarlett Johansson’s glamourpuss energises the story a little, but her frantic efforts to ape the accent (too Jersey Shore for words) is grating in unintended ways.

Too few films pick up so satisfactorily in their final acts. It is more rare still for a reformed protagonist (think American History X and Groundhog Day for counter-examples) to be as compelling as his earlier, more horrid self. But Don Jon properly comes alive when Don begins a spiky relationship with an older woman at night class.

Indeed, so effective is the odd chemistry between Gordon-Levitt and Julianne Moore that one ends up wishing the story had taken up the entire picture. Let's hope the actors kept one another's phone numbers.