Internet porn, the empathy void, growing through the love of the right person and admitting that you like Marky Mark's Good Vibrations - all these and more in Joseph Gordon-Levitt's directorial debut Don Jon, which is in cinemas now. Harry Guerin reviews.

When an actor has worked with filmmakers of the calibre of Christopher Nolan, Rian Johnson and Robert Redford, among others, it's no wonder that they want to give directing a go themselves. And so Joseph Gordon-Levitt steps behind (and in front of) the lens with his own script for a feature directing debut that starts off as one kind of comedy-drama, only to become a far more moving experience as the story progresses. There's a lot of crudity in Don Jon, but a lot of heart too.

Jon Martello (Gordon-Levitt) is a New Jersey alpha male who lists the important things in his life as "my body, my pad, my ride, my family, my church, my boys, my girls, my porn". Mostly the porn. While Jon, it seems, can pick up (then discard) any woman he wants, none of them interests him quite as much as videos of the ones he can find online.

Into this hamster wheel world of (fairly) instant gratification and unacknowledged yet intense loneliness walks Barbara (Scarlett Johansson). Jon reckons she is the best-looking woman he has ever seen, a beauty which is only enhanced when she gives him the brush-off.

Jon, of course, persists with his latest quarry, convinced that Barbara can complete the picture of his otherwise 'perfect' life - and perhaps offer more to him physically than the laptop.

For the first half of Don Jon there's the feeling that it's a movie for 16-year-old boys, co-directed by a 16-year-old boy as Gordon-Levitt becomes too keen on emphasising his character's porn addiction when you get the idea after one montage. The mechanics and conclusion, after all, don't change.

Gordon-Levitt's choices for the character mean that, early on, we don't care about Jon - a less likeable but no less lost Tony Manero from Saturday Night Fever - as much as we should. Instead, it's the other roles he has written, and the actors he has cast to play them, who are more satisfying. Johansson is excellent as the woman with the broadest New Jersey accent imaginable (and the steeliest determination to mould a man into who she wants him to be), while Tony Danza, as the father who proves that the apple doesn't fall far from the tree, makes you wish that we could see more of him in movie roles.

Then Julianne Moore appears and suddenly everything starts falling into place.

Playing Jon's stoned and chaotic night school classmate, Moore's performance and Gordon-Levitt's later writing bring the film from the salacious to the sublime, turning raunch into real life in all its magic and messiness. The scenes between Moore and Gordon-Levitt are performance and pitch perfect; if you want to see two actors really bringing out the best in each other it happens in a couple of scenes here.

Frustratingly, and like Jon's keyboard adventures, everything ends too quickly - the credits roll and you're left feeling a bit like one of Jon's conquests. Having succeeded in so many areas as a writer-director, there's a big lesson here for Gordon-Levitt for the next time. And there will be a next time: Don Jon was made for $6m and took nearly $30m. That's someone you do call back.