Jonah Hill made a big positive impression in 2007's "Superbad," but that was a high school comedy, and it is not always clear if actors will be able to successfully graduate from adolescent roles to adult ones. "Get Him to the Greek" deserves to be Hill's breakthrough, then—proof that he has what it takes to become a leading man, no matter how unlikely that may seem by Hollywood's standards. Hill isn't classically handsome, nor does he have six-pack abs. Like Seth Rogen before him, he's an ideal everyman, the kind of very funny but endearing and identifiable performer whom audiences feel as if they can easily connect to. In a film that is scattershot, more often than not less fun than its makers believe it to be, Jonah Hill is the one reliable constant, making the raucous trip worth taking and the destination more emotionally true than one could possibly have guessed.

When Pinnacle Records' expletive-spouting boss Sergio Roma (Sean Combs) demands some fresh ideas to enliven the company and garner more promotion, lowly intern Aaron Green (Jonah Hill) suggests holding a tenth-anniversary comeback concert at the Greek for British rock god Aldous Snow (Russell Brand), a musician whose star has fallen ever since the release of his venomously received single "African Child." Sergio likes the idea, and suddenly Aaron is on the road, sent to pick up Aldous in London, escort him to a Today Show appearance in New York City, and get him back to Los Angeles for the concert in the span of 72 hours. Seems easy enough, right? Not when you're dealing with an out-of-control celeb happily in the midst of a drug bend.

Written and directed by Nicholas Stoller, "Get Him to the Greek" is a spin-off of 2008's "Forgetting Sarah Marshall" that positions the cheerfully pretentious Aldous Snow—a supporting player in that earlier film—as a more fleshed-out main character. Russell Brand (2008's "Bedtime Stories") reprises the part, though, whereas he was a scene-stealer in "Forgetting Sarah Marshall," here he occasionally wears out his welcome. Like a little child who is difficult to control and just so happens to be into hard drugs, prescription pills and booze, Aldous tends to wear one's patience thin with his antics. Then again, that's kind of the point, with Aaron Green the outsider protagonist who gets a three-day glimpse into Aldous' life and, much to his own surprise, ends up living it himself. Torn between his responsibilities at work and letting loose—he has just semi-broken up with his live-in girlfriend Daphne (Elisabeth Moss)—it is Aaron's personal journey toward ultimately recognizing what it is he truly wants out of life where the film excels beyond that of a broad slapstick.

Aldous' faltering career—"African Child" is described as the worst thing to happen to that country since Apartheid—and the rocker's sense of entitlement are good for some sharp-pointed music-industry satire. A disastrous stop at the Today Show, heroin smuggling on an airplane, and a visit to Las Vegas where Aldous reconnects with his father (Colm Meaney) and Ethan is sent on a perilous search for drugs pass by in episodic fashion, some more amusing than others and most relying a bit too heavily on scatologia. If the pacing and gags are both uneven, the picture improves considerably as it moves into its third act, first with a wildly failed attempt at a ménage a trois, and then with a decided shift toward the more serious. Clear-headed for the first time in who knows how long and with his concert not even hours away, a reborn Aldous rises from a leap into a swimming pool and tells Ethan, "I feel nervous. It feels good to feel something." A line like that, if delivered incorrectly, could get bad laughs, but Russell Brand, shed of his hyperactive tendencies, sells it as he approaches something verging upon pathos. Likewise, the relationship between Ethan and Daphne, never less than authentic and textured, works itself out in natural, exceedingly sweet ways. Their romance is one built on a maturity, trust and soulfulness that raucous studio comedies of this sort rarely hold.

Jam-packed with cameos (including one from Kristen Bell, again portraying Sarah Marshall, that earns the film's single biggest laugh), "Get Him to the Greek" is better in spurts than as a whole. If Nicholas Stoller's script and direction aren't always air-tight, leave it to the actors to buoy the material up a much-appreciated notch. In addition to Jonah Hill's wonderful presence and Russell Brand's self-deprecation, Elisabeth Moss (2009's "Did You Hear About the Morgans?") brings both levity and sincerity to Daphne, while Rose Byrne (2009's "Knowing") is a comic revelation as Jackie Q, an airheaded pop star who could give Anna Faris' similar character in 2005's "Just Friends" a run for her money. The dud in the ensemble is Sean Combs (2001's "Monster's Ball"), unappealing as the abrasive Sergio Roma. Combs goes over-the-top to no avail; he's playing someone who isn't funny, just obnoxious. Fortunately, his role is small. Culminating in a few self-realizations, a cheering concert, and some really catchy tunes, "Get Him to the Greek" ends on a feel-good high note suggesting one mustn't always judge a book by its cover. The good will brought about by the strong finish is plentiful, but Jonah Hill's charismatic performance—playing someone you'd like to get to know and be friends with—ensures that it's worth the wait.