Funny People, Judd Apatow, 146 mins, (15)

Humour – cinema so often reminds us – is no laughing matter. When comedy is itself the subject of a film, it's invariably associated with depression and moral vacancy. Take The King of Comedy, in which Jerry Lewis seemingly played himself, the clown as jaded curmudgeon; or Billy Crystal's Mr Saturday Night, about a comic who brings grief to those around him; or Funny Bones, an intensely strange British film in which humour is depicted as a congenital curse.

Now comes Funny People, Judd Apatow's follow-up to his charming hit Knocked Up. Funny People is Apatow's equivalent to the album by a band that makes it big, then fills their next record with laments about the futility of success: it's his lonely-at-the-top movie. However, Apatow isn't moaning on his own account but depicting the quandary of successful people he knows – in particular, his old friend Adam Sandler.

Sandler has made more duds than any Hollywood comic bar Robin Williams, Eddie Murphy and Steve Martin. But while those actors' careers have been tragic, because we know how brilliant they once were, Sandler's genius is still a matter of conjecture. The film begins with old footage of Sandler himself as a young, hungry joker making strident prank calls. People in the background crack up, but Sandler's shtick isn't that funny – perhaps even less than his films Happy Gilmore or Big Daddy. But Apatow's point is that back then Sandler was innocent: he loved comedy.

In Funny People, Sandler plays a version of himself: comedian George Simmons, who has made a fortune starring in lousy films like Merman, in which he wears a fishtail, and one in which he competes in a hot-dog eating contest while a young boy pleads, "Dad, this won't bring Mom back!" Where George was once a joyous brat, his face now hangs leadenly and he lives a solitary, loveless existence in a Malibu mansion. One day, George is diagnosed with almost certainly fatal leukemia. To quote Max Wall, the comic master of despond: it's got to be funny, hasn't it?

Hoping to retrieve his mojo in the face of despair, George does an impromptu club gig and baffles the audience with his staring-into-the-abyss ramble. Aspiring stand-up Ira (Seth Rogen), who goes on after him, salvages the situation by improvising some halfway decent material in response: "Tomorrow night, Robin Williams slashes his wrists on stage."

Consequently, George hires Ira as his assistant – writer, gofer, warm-up and buddy, since he doesn't have any real friends. Ira earns his pay by being the best pal a misanthrope could have: he is beautifully played by Rogen, whose adoration of George is vividly limned in his puppyishly open face. Barring one serious transgression, which Apatow never follows through dramatically, Ira is the film's righteous man, maintaining his sanity while those around him act out of delusional egotism. Inevitably, there are lessons to be learned by Ira and George, but not in obvious ways. Discovering friendship and pleasure, George gets a new lease of life – then becomes even more obnoxious.

While Funny People doesn't approach the genuinely self-puncturing showbiz satire of Curb Your Enthusiasm, it is bracingly cynical about the rewards of success: women want to sleep with George for the pleasure of crying out, "Fuck me, Merman!" Yet it's never quite clear whether Apatow has it in his heart to be savage or whether, ultimately, he too is in the backslapping business. Amid numerous in-person cameos (Sarah Silverman, an irascible Eminem), a horrifying sequence has George playing at a MySpace convention, sharing the stage with James Taylor. MySpace founder Tom Anderson appears in person to gloat, "Everyone has their price": a nice jibe but to have the real Anderson say it, before a roomful of grinning delegates who resemble clean-cut cult members, kills the satire. You fear for Apatow that he genuinely likes the MySpace people, or worse, James Taylor: that's just not compatible with a sense of humour.

This over-long film goes off the rails when George seeks a reconciliation with his ex-girlfriend Laura. This final act, with Eric Bana nicely spotlit as her blowhard husband, lets go of the premise of a self-enclosed comedy universe: it becomes a middling Woody Allen farce about spoilt rich folk. Laura is played by Leslie Mann, Apatow's wife; their daughters Maude and Iris make another appearance as Laura's kids, goofy and adorable as ever (but boy, are they going to resent Dad's movies one day). Mann and Aubrey Plaza, as Ira's nerdy-hip object of adoration, have the only substantial female roles; but while Plaza gets to swipe smartly at male presumption, Mann's Laura has nothing like the abrasive force of her character in Knocked Up. Apatow still doesn't quite know how to write about, or for, women. But he knows about guys, at least comedian guys: if their default mode of humour is the penis joke, so be it.

Despite the lack of discipline, Apatow is trying to stretch himself, to say something about human nature: his insight here is less about comedy than about adult life as a succession of equivocations and betrayals. Sandler pushes himself too: utterly convincing as a world-weary sourpuss, he has a great scene in which George performs a mock-maudlin song that turns into a black litany of hatred, of himself and of his audience. Yet the surprise is just how positive about humour Apatow finally manages to be: the film's closing shot has two guys in a supermarket swapping gags while blank-faced shoppers cruise the aisles around them. If that isn't an image of Funny People in relation to most bland, interchangeable Hollywood comedy, my name's Happy Gilmore.

Also Showing: 30/08/2009

Mesrine: Public Enemy No 1 (130 mins, 15)

The second of this month's thrillers about France's most notorious bank robber, Mesrine: Public Enemy No 1 covers the 1970s, as Jacques Mesrine (Vincent Cassell) devotes as much energy to winding up the police and courting the media as he does to his heists. If you haven't seen the first instalment, don't be put off. On its own, this one offers some of the most kinetic car chases, jailbreaks and shoot-outs since Jimmy Cagney was on top of the world.

The Final Destination (82 mins, 15)

Digital 3D is fast becoming a multiplex fixture, but so far no one has come up with a better use for it than hurling sharp objects and severed body parts at the audience. The Final Destination does so with such gusto that you can almost forgive its disingenuous title. "Final Destination 4" would be more honest.

Jetsam (84 mins)

Zero-budget British thriller which has a preposterous hi-tech espionage plot, despite looking as dingy and grey as any kitchen-sink drama.