JUDD APATOW and his entourage have made one great comedic innovation: They discovered that the only thing funnier than a male butt-shot is a dick-shot. Forgetting Sarah Marshall may be best remembered for its ad campaign, which has inundated whole cities with taunting graffiti. But the full-frontal shots—amping up the vulnerability and silliness of a naked man—get the yuks.

This Apatow-produced, Nicholas Stoler–directed film comes across as a kind of adult big-baby fantasy with everything but the Pampers. Scriptwriter Jason Segel plays Peter (it figures), a child-man who is involved with a television star, Sarah Marshall (Kristen Bell), who appears in an autopsy show called Crime Scene. When she comes home and finds Peter waiting for her naked, she breaks up with him with no fanfare, even though they have been together for five years.

Grieving horribly, Peter flies to Oahu to flee his heartbreak and stumbles into the same resort where Sarah is vacationing with her new man, Aldous, a moronic British rock star (Russell Brand, the best thing in the picture). Although Peter doesn't have the money to stay at the resort, the smoky concierge, Rachel (Mila Kunis), decides on the spot to let Peter stay for free in one of the empty bedrooms, which is within hearing distance of Sarah and Aldous' love nest. Rachel is willing to be dated, too, but the foolish Peter can't let Sarah go.

What follows is a series of disconnected sketches about the uncomfortable meetings between the ex-lovers. The scenes have a rhythmless rhythm, as if they were the result of longer improvs and had been cut down brutally.

The seriously poor photography of Hawaii is another drawback. The filmmakers couldn't help the weather, which was overcast and blowing throughout the film, but they are to blame for trying to fix it by digital dying the ocean to a bright, false swimming-pool blue.

Then again, the poor photography matches the smaller-than-life performances.

The cast of TV actors, blown up to big-screen size, looks about as good as blowing up 8 mm to 35 mm. The cast is large and busy and seems to be there to overcome the thinness of the script.

You can always tell—even if you don't watch a lot of television—when a sitcom star is making an entrance in a movie. It's not that they do anything; its just that the director holds the shot, waiting for the audience's delighted recognition. That's about the size of Bill Hader's podcasted-in performance on Peter's laptop. The same goes for 30 Rock's Jack McBrayer as a virgin Mormonoid on his honeymoon. (He does the old frat-house line about "if God were an urban planner, he wouldn't put the playground so close to the sewage-treatment plant.")

Jonah Hill, from Superbad, plays a fawning waiter who begs Aldous to listen to his demo. Hill is a low, low comedian—the one in Apatow's universe who is best at linking dick humor with a hurt, sensitive side. But Hill has no more purpose than to be the hired help who gets above his station in life.

Steve Landesberg, another familiar TV face, turns up as a doctor, whose advice to Peter is "Use your dick." In Apatowland, that's how you survive. A man can say something outrageous to a woman, and she's a good sport about his honesty. But there is little reciprocation. When Peter follows his doctor's advice by bedding a chain of women, they talk too much or too little, but they aren't saying anything (one even asks for a bondage gag when she's on her back).

We never learn what Peter saw in Sarah, besides the fact that she's desirable. We never learn why she was in love with Peter for five years but could suddenly drop him cold without any subsequent meetings. Later, in flashbacks—retrieved post-traumatic-syndrome memories—Peter recalls some cruel or foolish things Sarah did over the years.

The scene is supposed to mean that he is healing. In fact, now we understand even less about why Peter and Sarah were a couple. Since he's a man-child who gobbles metal salad bowls full of generic Froot Loops, does imitations of Ian McKellen in The Lord of the Rings and has a thing for puppets, we can guess why she wanted someone more mature. Or mature looking—as if Aldous were mature, with his clashing spiritual tattoos.

The title character is the vaguest person aboard. Bell is no more a natural comedian than she is a natural romantic lead. But is it her fault? She has no character here to speak of. She is in this movie, just as the billboards suggest: strictly for the purpose of being told off.

FORGETTING SARAH MARSHALL (R), directed by Nicholas Stoller, written by Jason Segel, photographed by Russ T. Alsobrook and starring Peter Bretter and Kristen Bell, opens April 18 valleywide.

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