Johnny Depp has clearly performed a labor of love in not only excavating Hunter S. Thompson’s early semi-autobiographical novel from manuscript obscurity but shepherding it to the screen and taking the lead role itself. But while its quirky, over-the-top tone elicits chuckles and even a few belly laughs, “The Rum Diary” runs out of steam as it proceeds, fading like a drunk on his last legs.

The Thompson surrogate is Paul Kemp (Depp), an alcoholic would-be novelist who arrives in Puerto Rico in 1960 to take a job as a staff reporter with the San Juan Star, a tabloid run by perpetually exasperated editor Lotterman (Richard Jenkins). He soon falls in with the paper’s smart-talking, dissolute photographer Sala (Michael Rispoli), who shares a room with an even odder soul, sloshed, bohemian Moburg (Giovanni Ribisi) and quickly adds himself as a third roommate.

The plot is an episodic affair in which Kemp survives multiple wacky adventures, but its most important element, between crazy bits with Sala and Moburg and confrontations with Lotterman, is his involvement with Sanderson (Aaron Eckhart), a slick real-estate developer who wants to involve him in a shady deal that would turn a nearby island just about to be decommissioned as a firing range by the Pentagon into a tourist mecca, complete with luxury hotel. The prospect upsets Kemp’s addled sense of propriety, but also gets him close of Sanderson’s live-in girlfriend Chenault (Amber Heard), a sultry blonde with a wild streak. Unfortunately, things go sour for Kemp both with Sanderson and at the paper, and he winds up jobless and possibly heading for a long jail term, so he plans an escape from the island.

“The Rum Diary” offers some amusement, particularly in the hard-boiled dialogue assigned to Kemp, Sala and Lotterman, which is reminiscent of the sort of stuff that filled pictures in the 1940s, though absent the obscenities. And it certainly has some funny bits of business—like the way Kemp and Sala get the latter’s ruined car home. But despite its best efforts, the talented cast can’t sustain the picture over the long haul. By the final reel it’s fallen apart as completely as Kemp’s life has, and is just spinning its wheels.

Still, Depp has as much fun playing Thompson’s stand-in as he did playing the writer himself in “Fear and Loathing in Las Vegas”—though in both cases probably more than the audience will have watching him. Rispoli cuts an engaging figure as the world-weary Sala, and Jenkins makes the editor a rambunctious figure reminiscent of “The Front Page” (though the continuous bit about his toupee suggests that Thompson’s sense of humor at the stage was still pretty immature). On the other hand, Ribisi tries too hard as the peculiar Moburg, and Eckhart is just a blandly smooth villain. (Bill Smitrovich is better as a crude anti-Communist member of Sanderson’s group, with one scene that’s actually pretty shocking in today’s climate.) And while Heard is certainly a beautiful woman, the whole subplot dealing with her is one of the main factors in the picture’s loss of momentum.

“The Rum Diary” looks right, blending the locations with subtle period touches courtesy of production designer Chris Seagers, art director Dawn Swiderski, set designers A. Todd Holland, Luis Lopez Baquiero and Enid Soto, set decorator Rosemary Brandenburg and costume designer Colleen Atwood. And it’s shot in appropriately gritty style by Dariusz Wolski. Christopher Young’s score adds an appropriately tropical air, too.

But writer-director Bruce Robinson seems unable to capture the full flavor of the source material in either capacity. The writing isn’t as reliably sharp as it should be, and his choreography of the scenes often feels limp, which is why Depp’s extravagant performance sometimes just turns sleepy. In the end this “Rum” lacks the punch it should have had.