The Aldous factor.(FILM)

THERE'S only one reason to see Forgetting Sarah Marshall, and his name is Aldous Snow. Or maybe I should say Russell Brand, since the line between the actor (Brand, that is) and the character he plays is not exactly clear. Snow, the lead singer of a band called Infant Sorrow, is the musician for whom the Sarah Marshall of the title dumps the movie's hero, and he's a vain and utterly hilarious popinjay--a long-haired, hip-waggling, eyeliner-sporting, dead-on parody of dozens of interchangeably self-absorbed pop stars.

The real-life Brand, meanwhile-a British celeb just beginning his inevitable invasion of our shores--is, well, a vain and hilarious popinjay, whose entire career looks like a long send-up of the famous-for-being-famous crowd. Floated on little more than charm, outrageous outfits, and a flair for deadpan one-liners, he's made his way as a radio and television host, a newspaper columnist (British papers are rather more fun than ours), a sketch comedian, and the author of a tell-all memoir about his various addictions (sex, drugs, etc.) titled My Booky Wook. His turn as Snow, then, isn't exactly a stretch, but it's a worthy addition to his resume, and I would almost suggest buying a ticket to Forgetting Sarah Marshall just to see him perform Infant Sorrow's hit single "We've Got to Do Something," a glorious parody of the "We Are the World" school of politically engaged pop music.

Fortunately, though, you can probably find the video on YouTube and spare yourself the film, which is unworthy of both his gifts and your attention. Marshall is the latest production from the Judd Apatow comedy factory, which has turned out delights like The 40-Year-Old Virgin and last year's Superbad, but which of late seems to be entering the period of rapid decline that all comedic posses eventually fall into. It's a sociological problem as much as an artistic one: When you hit it as big as Apatow has, the natural thing to do is help your friends and associates (and their friends and associates, and so on) hit it big as well. This means, at first, giving breaks to people who richly deserve them; then to people who sort of deserve them; and finally to people who don't really deserve them at all.

Thus the mediocrity of last Christmas's Walk Hard and the awfulness of March's Drillbit Taylor. Thus, too, Forgetting Sarah Marshall, a forgettable (sorry, couldn't help it) serving of comedy written by and starring Jason Segel, and directed by Nicholas Stoller--neither of whom, unfortunately, is up to the task of carrying off even as lightweight a piece of cinema as this. Segel has done solid supporting work for the Apatow machine in the past--in last summer's Knocked Up and the short-lived TV shows Freaks and Geeks and Undeclared-but as a ...