In "Get Him to the Greek," opening today and likely to amuse and aghast in equal measure, Russell Brand plays the same British rock 'n' roll prat — Aldous Snow — that he assayed so expertly in the 2008 romcom "Forgetting Sarah Marshall."

Only this time, Aldous is off the wagon and on any number of mind-warping chemicals. He's an obnoxious, narcissistic, groping, blotto nightmare of a ninny — and he's the "him" in the title of Nicholas Stoller's improbable, intercontinental buddy movie farce.

Aldous has to make it from his home base in London to Los Angeles for a big anniversary gig at the storied Greek Theater. Good luck to Jonah Hill, playing the dweebish record company underling with the job of accompanying Aldous — and babysitting Aldous, and smuggling drugs for Aldous — on his jaunt.

"In 'Sarah Marshall,' Russell's character strangely represented the moral high ground," says Stoller, who directed Brand in both "Forgetting Sarah Marshall" and "Get Him to the Greek." "But here, he's more like the moral low ground."

And Brand plays the moral low ground in 3-D: dark, debauched, deluded.

Stoller says that the idea to bring Brand back — and pair him with Hill — came in the early stages of making "Forgetting Sarah Marshall." Hill plays a different, and less central character, in that film, but "during the table read, I looked at Russell and Jonah, and they had really good comic chemistry. I thought, the two of them, that's a movie. I don't know what kind of movie it is, but that's definitely a movie."

Later into the production of "Sarah Marshall," which starred Kristen Bell in the title role and Jason Segel as the guy trying (unsuccessfully) to forget her, Stoller came up with the "Greek" scenario — a wild, weird, make-the-deadline-or-else romp. Initially, Stoller had Brand playing a different English rock star, but that didn't make any sense.

"So we made it a spinoff," Stoller explains. "People really liked the Aldous character and it's fun to explore a different facet of that same character."

Different facet, indeed.

When Stoller began writing the new project, he picked up the British comedian's autobiography, "My Booky Wook: A Memoir of Sex, Drugs and Stand-Up."

"Russell has been clean for a long time now, almost 10 years, I think, but when I read his book, I literally did not know the Russell I was reading about. He just seemed a totally different person."

And so Stoller sat down with Brand to see how he could accurately portray — but also comically portray — this alcohol-and-drug-addled rocker.

"I don't have experience with addiction, and I wanted to make sure it all felt real," he says. "The movie, while it's certainly a big comedy, it still tries to explore some serious themes, and we're hopefully getting into this character who's lonely and an addict. I wanted to make sure it was truthful, so I interviewed Russell and asked him lots of questions.

"There's nothing in the movie that's from his life, per se, but in terms of emotional truths and that sort of thing, he really helped me a lot with trying to figure the character out."

Brand also gave Stoller some advice on how to smuggle a packet of heroin onto a jetliner, but you'll have to see the R-rated film to get the rather graphic, and icky, details.

Stoller — like Segel and Hill — is part of the Judd Apatow comedy collective, an unofficial assembly of writers, directors and actors who trade in a brand of shock comedy that is also smart comedy, comedy grounded in real human behavior. Stoller wrote for Apatow's short-lived early 2000 TV series, "Undeclared," and Apatow produced, and contributed story elements, to both "Sarah Marshall" and "Get Him to the Greek."

Also contributing to the latter was Sean Combs, P. Diddy himself, who is inspired and hilarious as Hill's boss, the head of the record label that's trying to revive Aldous' career, and sell some music in the process. Combs has one particular soliloquy, about messing with other people's minds (although messing is not the word he chooses), that easily counts among the film's most outrageously funny scenes.

"Sean Combs improved that during rehearsals," Stoller recalls, "and I was like, 'That's the most incredible thing I've ever seen!' So, I wrote it into the script "