BY GLENN WHIPP >FILM CRITIC

"Forgetting Sarah Marshall" begins and ends with its star's private parts, a sign of the movie's groin-centric humor and its nakedly emotional vulnerability.

Even if you missed the opening credits, you'd recognize "Sarah Marshall" as the latest comedy from the Judd Apatow laugh factory.

Its lead actor and screenwriter, Jason Segel, worked with Apatow on "Knocked Up" and "Freaks and Geeks" and shares his mentor's belief that great comedy comes from humiliation and pain.

The sure-footed "Sarah" will be remembered primarily for Segel's bravery in baring all to the camera when the title heroine (Kristen Bell) summarily dumps his character, Peter. Initially, Segel's full-frontal nudity is played strictly for laughs, but as the scene keeps going (and going), and Segel remains naked, we viscerally feel the rawness of Peter's dejection.

Not that either Segel or director Nicholas Stoller is wont to linger on touchy-feeling emotions. "Sarah Marshall" has a strong eye for character, but it exists to make you laugh, not to reveal truths about the human condition. And the movie is ably abetted by the presence of Apatow regulars like Paul Rudd, Jonah Hill and Bill Hader in supporting roles.

After the breakup, Peter heads to Hawaii for a little therapeutic r&r, a plan that goes horribly and hilariously awry when Sarah shows up at the same resort with her new boyfriend, an oversexed English rock star, Aldous Snow (Russell Brand).

The situation opens up an embarrassment of embarrassments for Peter, but the movie subverts expectations by making Sarah human and the rival boyfriend a likable cad.

Peter himself is kind of weird - he's lazy, weepy and way into puppets. You can see why Sarah showed him the door. You can also see why the resort's beautiful desk clerk (Mila Kunis) might find his eccentricities kind of endearing.

There are echoes of "Knocked Up" and "Superbad" here, but the appealing Segel has invested the story with enough personal details to make it feel like an outgrowth of his fabulous television work on "Freaks and Geeks" and "Undeclared." This is his first produced screenplay, but the material was a lifetime in the making.

glenn.whipp@dailynews.com
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"Forgetting Sarah Marshall" originally ended the way most romantic comedies do: The guy gets the girl. Cue credits.

Producer Judd Apatow told the movie's writer and star Jason Segel that he needed to come up with something different, something unique.

"So I said, almost as a joking challenge, `What about a lavish puppet musical?' " Segel remembers. "He said, `That is the weirdest thing I've ever heard, and it could be really funny.' "

That was the beginning of the movie's hilarious climactic scene, an elaborate, operatic Dracula musical titled "A Taste for Love" that features 30 puppets created by Jim Henson's Creature Shop. The puppet Dracula musical not only gave Segel a singular ending for "Sarah Marshall," but it also led to his next job. He and "Sarah Marshall" director Nick Stoller are currently writing the next Muppet movie for Disney.

Says Segel: "I was in a general meeting at Disney, and people were pitching movies to me … which was crazy enough by itself. So I just worked up my courage and said, `Listen, I gotta be honest with you. The only thing I want to do with you guys right now is write the new Muppet movie.'

"And there was a smattering of laughter. `Oh. Funny. Good one.' `No, I'm serious. And here's the idea.'

"And I pitched it, and they bought it in the room. Crazy. I still think they're confused."

Segel, a puppet nut since he was a kid, says the film will be an "old-time Muppet movie" where Kermit, Miss Piggy and the gang will put on a musical show to save the studio. Expect cameos from Apatow regulars and - if Segel can persuade him - Charles Grodin, who starred in 1981's "The Great Muppet Caper."

>Glenn Whipp

FORGETTING SARAH MARSHALL

R: sexual content, language, some graphic nudity.
Starring: Jason Segel, Kristen Bell, Russell Brand.
Director: Nicholas Stoller.
Running time: 1 hr. 52 min.
Playing: Area wide.
In a nutshell: Latest from the Judd Apatow laugh factory goes after the naked truth in relationship breakups.