Kevin Smith has come a long way since he stunned us all with the fearlessly frank Clerks. And mostly backwards, if this vulgar comedy is anything to go by. It’s vulgar for two reasons — and by vulgar, I do not mean that it is in any way shocking.

The first is that it uses words and situations that can’t be printed in this newspaper simply for effect rather than as a natural part of its characters’ lives. The second is that, having painted the principal members of his cast as oafs, Smith then spends the final half-hour changing gear into an equally fake kind of sentimentality that seems to be saying: “I didn’t really mean it, folks.”

It stars Seth Rogen, an actor capable of much better — as in Knocked Up and Superbad — as Zack, and the attractive Elizabeth Banks as Miri. They play sexually uninvolved flatmates whose water and electricity have been cut off for non-payment of bills. She has to rinse her hair in the lavatory. Desperate to make some money, they hire a ragbag of struggling actors to make a pornographic film in Pittsburgh.

Most of them, not all professionals, are perfectly willing to go to it. But the script, such as it is, requires Zack and Miri to “perform” with each other and, lo and behold, when they finally get rid of their inhibitions and have sex they realise they are actually not just friends but in love. Hence the sentimentality which clothes the movie in a different kind of slush.

It is not all dross. I have to admit that there are some good jokes and a few situations that will make you smile. There is, for instance, a nice scene where Miri, determined to get laid at a party, ogles a handsome would-be beau only to find that he’s gay. The ensuing quarrel between him and his lover is both funny and well observed.

The other stars of their film are well played and anyone who has seen porno films will recognise the absurdity of it all. But it still leaves a slightly brackish taste in the mouth — it is not only Russell Brand and Jonathan Ross who are capable of going astray in comedy — this joke movie is essentially laddish and juvenile too.

No doubt it will be as popular as the Brand and Ross prank in some circles and if it doesn’t make money I’d be very surprised. But that doesn’t make me like it any better. If this is part of our comedy culture, there’s definitely something wrong somewhere.