Forgetting Sarah Marshall 

At this point, Judd Apatow isn’t so much a writer, director or producer as a brand. Regardless of his role in a project, you pretty much know what you’re going to get: a trademark mix of the raunchy and romantic, in which a rotation of supporting regulars does at least as much comic lifting as the leads.

In the case of Forgetting Sarah Marshall, Apatow plays the producer. This job perhaps involved little more than giving first-time director Nicholas Stoller Jonah Hill’s cellphone number. The film was written by its star, Jason Segel, who previously did supporting slacker duty in Knocked Up and costarred in the Apatow-produced television series “Freaks and Geeks.”

Segel is an unlikely lead, even in this self-contained universe of child-men. To describe him as Judge Reinhold-esque would risk overstating his charisma and raw, masculine appeal. He plays Peter Bretter, the composer for a popular “CSI”-style show whose star (Kristen Bell) he’s been dating for five years. Early in the movie she calls to say she’s back in town and needs to see him right away. Being a child-man, Peter tidies up the place by tossing dirty clothes and pizza boxes into a closet and showers in anticipation of a steamy reunion.

Instead of a roll in the hay, however, he is treated to a heave-ho. The sequence is notable for the fact that Peter receives the bad news while stark-naked, fresh from his ablutions. Apatow recently declared his intention to work a penis into every picture with which he’s associated (everyone in Hollywood has a cause), and Peter’s peter is the star of this scene. Its presence does make for an awkward, cringy moment or two, especially when he insists on embracing his fully clothed ex. But the stunt is really less about shock value than about distracting the viewer from writing and acting that are otherwise break-up boilerplate.

Things take a turn for the somewhat more entertaining when the heartbroken shlub concludes that the only way to get his mind off Sarah Marshall is to board a plane for Hawaii. Naturally, Sarah turns out not only to be on her own simultaneous Hawaiian getaway, but to be staying at the very same hotel as Peter — with her new boyfriend. And, naturally, her boyfriend (Russell Brand) is not just cooler and better-looking than Peter; he’s also a more successful musician — a British rock star, in fact.

A great many oddball flourishes, zigzaggy plot developments and Apatow-quality laughs are required to compensate for contrivances this shameless. Thanks to its supporting cast, the film delivers just enough of them to make it marginally recommendable. Peter is befriended by an attractive hotel receptionist named Rachel (Mila Kunis), for whom he falls once he finally stops weeping in his room — so loudly the other guests complain. Will he hook up with Rachel, or somehow get back together with Sarah? Honestly, neither Segel nor his script provides sufficient reason to care.

Fortunately, the film offers more rewarding side stories and subplots. The aforementioned Jonah Hill is a hoot and a half, as usual, in the role of a hotel waiter with a man-crush on the rocker, Aldous Snow. Paul Rudd is casually hysterical as a surfing instructor whose brain is so weed-fried that he takes clients into the deep water, gets to talking and forgets to surf. There’s Bill Hader, whose video-cam communiqués with Segel are priceless mini-movies-within-the-movie. And “30 Rock”’s Jack McBrayer is fabulous as a super-Christian newlywed freaked out by his bride’s sudden lustiness.

But Brand’s Aldous Snow is easily the film’s most consistently interesting creation. Cutting a figure that suggests Jimmy Page in a Speedo and flip-flops, he tosses off some of the picture’s pithiest lines with a divine world-weariness. The interplay between the rocker and the jilted noodler is more compelling by far than any of the relationships between the movie’s male and female characters — particularly Segel and Bell, whose interaction doesn’t ring true for a single second of the production’s leisurely hour-and-50-minute running time. The combined talents of the picture’s quirky secondary players have an unintended effect: They make the fact that Peter and Sarah are even on the same island eminently easy to forget.