If Jason Segel hadn’t written “Forgetting Sarah Marshall” himself, it would be easy to assume that he had made some extremely compromising promises on the casting couch to win his role in the film.

Bookended by full-frontal shots of Mr. Segel in the buff, “Forgetting Sarah Marshall,” the latest in a seemingly unending series of comedies associated with Judd Apatow (he serves as producer here), navigates the indignities of a big breakup while providing enough romantic comedy elements to keep date audiences happy.

Most Hollywood actors might not envision themselves as the sniveling, weepy, frequently naked emotional wreck, yet this is how Mr. Segel wrote the character of Peter Bretter. Give credit where it’s due: The 6-foot-4-inch character actor apparently decided to take matters into his own hands, and wrote his own film after a decade of supporting gigs in popular movies and television shows failed to bounce him into lead roles. Inspired by a naked breakup he experienced firsthand, Mr. Segel’s script plumbs the depths of emotional breakdown with a good deal of comedic success.

After getting dumped by his TV star girlfriend, Sarah Marshall (Kristen Bell), Peter decides to escape his woes with a trip to Hawaii, where — you’ll never believe it — he runs into Sarah and her nymphomaniac rocker boyfriend, Aldous Snow (Russell Brand). Luckily, the despondent Peter meets Rachel (Mila Kunis), a cute hospitality worker at the hotel, who helps him begin to get over the collapse of his long-term relationship.

Mr. Segel might not have the marquee looks or idiosyncratic personality that propelled his “Freaks and Geeks” co-stars James Franco and Seth Rogen into the leading-man club, but he is a reliable talent whose unselfishness with his screen time elevates most any ensemble situation — a skill well documented every week on CBS’s “How I Met Your Mother,” on which Mr. Segel plays the character Marshall.

That generosity is on display throughout “Forgetting Sarah Marshall.” Mr. Segel wrote the script to showcase his own talent, but he also packed the film with up-and-coming comic actors who are used to varying degrees of success.

The script does its best with the main characters, veering away from shallow villains and infallible heroes. Oversized, weeping Peter is a lovable disaster, and his chemistry with Ms. Kunis develops well, making a case for the petite former “That ’70s Show” actress on the big screen. Ms. Bell does an adequate job as an actress starting to believe her own hype, but her scenes are fantastically stolen by Mr. Brand, who is absurdly charming as the caddish but undeniably zen Aldous. In particular, his character’s hypersexualized song, “Inside of You,” is disturbingly catchy.

In a smaller role, Paul Rudd (a regular player in the Apatow company) reaffirms his reign as comedic cameo king, playing a stoned surfer with short-term memory loss. But other bit characters do not fare as well. Many of the roles seem improvised, and a lack of direction leaves the normally well-timed Jonah Hill out at sea as a strange hotel waiter. Similarly, Jack McBrayer, who plays a rube with exquisite dedication on NBC’s “30 Rock,” is rendered an embarrassing stereotype here.

“Forgetting Sarah Marshall” takes more than a few cues from the gross-out humor that typifies Mr. Apatow’s work, but though it may not bridge much new territory, it does manage to accomplish its goal, which was to give some extra screen time to a cast of talented actors who sometimes get lost in the Hollywood shuffle.