Jonathan Romney on film: Couple of kids, two cars, just one problem with This is 40

This sequel to 'Knocked Up' is just an open-mic session for Judd Apatow's nearest and dearest

Smug hugs: Paul Rudd and Leslie Mann improvise madly in Judd Apatow’s This Is 40

American comedy mogul Judd Apatow seems to be an agreeable sort of guy, and he has a lot of friends who think so too. Many of them appear in This Is 40, gamely turning up to do cameos, improvise or simply hang out with the amiable Apatow, who's behind the camera, and his wife Leslie Mann, who's in front of it.

Pitched as a "sort of sequel" to Apatow's hugely successful Knocked Up, this film gives us more of that film's supporting characters, Pete and Debbie (Paul Rudd, Mann) and their adorable, scene-hugging daughters Sadie and Charlotte, played by Apatow and Mann's real progeny, Maude and Iris. It's understood that, since three out of these four actors regularly share their granola with the director, what we're getting is a comic version of life chez Apatow. The premise is that Pete and Debbie are both turning 40, and reacting with different kinds of panic. She's insisting she's 38, envying the younger sexpot (Megan Fox) who works at her clothing store, and deciding to institute a new regime of happiness and acceptance in the household – which actually means, of course, a new regime of anxiety.

Pete, meanwhile, is still acting like an overgrown teenager, the eternal condition of all Apatow males – staring up Fox's skirt when she's climbing a ladder, taking Viagra for novelty birthday sex and pursuing blokeish musical tastes that, to be honest, suggest a much older man. One of the better running gags concerns Pete's failing record label and its latest signing, venerable British rocker Graham Parker. The latter turns up and complains about his gout, then performs with his band, the Rumour, and no one comes. Parker may not be the future of popular music, but he's a good sport.

Everyone here is a good sport. Rudd and Mann submit to various types of clinical embarrassment, he spreadeagling to examine his piles with a mirror, she being mocked by a gynaecologist ("I know how old you are by counting the rings"). Friendly faces show up to offer a turn, or a cameo, or barely half a cameo. Jason Segel is a narcissistic personal trainer; Chris O'Dowd is Pete's colleague; Lena Dunham is just sort of briefly there, with tattoos. There's a lot of improv, some paying off, some not. Charlyne Yi does a deranged rant and it's ugly and incongruous, like a gratuitous jazz flute solo. Melissa McCarthy (from Bridesmaids) has a scene raging in the school principal's office, and it's just funny enough; we then get her in full improv flight over the end credits, and it's worth waiting for (although Rudd and Mann's corpsing kills the effect a little). This is not so much a comedy, even a free-form comedy, in the way Knocked Up was; more a comedy jam, an open-mic night for family and friends.

It's surprising how leaden, mean-spirited or just wrong some of the humour is. Rudd mocks an Indian doctor's accent and you wince. The McCarthy scene isn't funny because Pete and Debbie slip out of character: apart from being smug, they suddenly also turn mean and joyless. It's as if Apatow is experimenting with the characters being malicious, just to test their range.

Besides, it's hard to care much about the woes of this two-kid, two-car family with their suburban house seemingly designed to retain every drop of West Coast sunshine. Cinematographer Phedon Papamichael gives interiors that caramel glow that's the romcom equivalent of the green paint in hospitals: it's designed to calm you down and reassure you that you're in a safe environment. A lot of the film is about that relaxed feeling: apart from occasional outbursts of shrieking, the film drifts by in a benevolent stoned glow. And how it drifts – This Is 40 minutes too long.

A few salient points about family life are sweetly made – such as that the reviving power of a sex-and dope-fuelled weekend away will dissipate within 30 seconds of being reunited with your squabbling kids. Otherwise, the film often lapses into soft semi-melodrama. When Debbie lunches with her distant, neglectful father (John Lithgow), the film briefly forgets it's a comedy, and Mann's big-eyed anxiety ceases to be funny, or even that likeable. A marital moment-of-truth scene between Pete and Debbie goes weirdly silent, as if Apatow had suddenly sensed Ingmar Bergman's ghost passing through the room.

A couple of people make this film bearable: one is Maude Apatow as the now petulantly teenaged Sadie, whose fit of rage is simply the most bracing moment here. The other is Albert Brooks, playing Pete's cantankerous, feckless dad. This veteran comic has as many memorable lines in his scenes as there are in his face; he now looks like the part of Mount Rushmore that fell off in a rock slide.

As for Mann and Rudd, it's hard to dislike affable leads so cheerfully willing to forgo their dignity, but you tire of them as you would after a long weekend with perfectly pleasant people you don't really know that well. The next time the Apatows throw an at-home, some of us may choose to be out of town.

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