Thompson was no exception among young writers; the novel was only a seedling of what would eventually bloom in merciless attacks on the American Dream in “Hell’s Angels” and the narcotized paranoia of “Fear and Loathing in Las Vegas.” Talk-show host Charlie Rose rightfully harangued the writer in 1999 for the late-career cash grab.

Unfortunately, director Bruce Robinson — who hadn’t helmed a film since 1992’s “Jennifer 8” — adapted the script with too much reverence, twisting what was an expression of Thompson’s fear of old age into a miscast romance with as many plot holes as swigs of alcohol.

Those expecting Raoul Duke instead get Paul Kemp, a lifeless Johnny Depp (“Pirates of the Caribbean: On Stranger Tides”) who — nearing 50 — looks nearly every second of it next to 25-year-old Amber Heard, whose Chenault sizzles like the Caribbean sands at noon in a summertime heat wave. Too bad she can’t convince us to consider her anything more than a petulant, impulsive thing in tight dresses.

“I just want some apple-blossom lipstick and fucks,” Kemp says after watching her copulate with an overtanned Sanderson (Aaron Eckhart, “Battle: Los Angeles”) from afar. The dialogue clashes with Christopher Young’s score, which insists on blasting flutes every time Heard and Depp go puppy-eyed.

Robinson relies too heavily on Thompson’s writing style (“Too many adjectives!” Kemp’s editor complains) to hold the script together, fails to develop the plot (Kemp’s search for “his voice”) and wanders down so many subplots (all starring “Avatar”’s Giovanni Ribisi at his most annoying) that he has to turn to inner monologue to pull the narrative together.

The only spot-on casting here is Eckhart’s slick PR stereotype. The white linen-sporting Sanderson greases the wheels of a lucrative hotel deal on a nearby island, thereby providing a villain, although the performance is really just a douchier version of his lovable, flawed “Thank You for Smoking” hero.

Thompson enthusiasts surely will enjoy themselves, catching a refracted view into the father of gonzo journalism’s early days, as well as an occasional gem or two (“He’s gotta mouth like an AP wire” and “blackheads like Braille”) amongst overwrought duds (“Your tongue is like an accusatory giblet!”).

And while Thompson did eventually “find his voice,” as Kemp hopes in the film, it’s a wonder he ever did.