Rude, raucous and gleefully violent, playwright Martin McDonagh’s follow-up to his debut feature “In Bruges” is sophomoric in every sense. But though insubstantial and childishly over-the-top, it’s also enjoyable, even though you might feel a bit of queasiness accompanying your laughter. What else would you expect of a movie with a title like “Seven Psychopaths”?

The fulcrum of McDonagh’s mess of a plot, set in L. A., is Marty (Colin Farrell, a holdover from “Bruges”), an alcoholic Irish screenwriter facing a monumental case of writer’s block: all he has is a title, which happens to be the film’s. Fortunately, or not, his wacky buddy Billy (Sam Rockwell) lends a hand by placing a classified ad inviting ex-psychos who might want to get their stories before the public to come by. The ad brings one contributor—Zachary (Tom Waits), a gloomy Gus clutching a rabbit whose tale of hunting down serial killers with his partner catches Marty’s fancy.

But Billy has troubles of his own. He’s collaborating with Hans (Christopher Walken, amping up his patented weirdness to an eleven) in a scam that involves kidnapping dogs and then returning them to their owners in hopes of a tidy reward. Unhappily Billy makes the apparent mistake of taking the beloved Shih Tzu of a deranged mob boss (Woody Harrelson) who will stop at nothing to get his pooch back. Lots of blood will be spilled and corpses created in the ensuing mayhem, though it’s punctuated by a good deal of McDonagh’s stock-in-trade conversations, filled with comically existential reveries, spasms of slick soul-searching and blustery pop-cultural riffs.

There’s really not much of a center to the script, which acts fundamentally as a skeleton on which the writer-director can hang his verbal set-pieces and oddball sketches. There’s a hint of an ultimate purpose behind everything, a sort of extreme wake-up call for Marty that might shake him out of his lack of productivity; that involves another psycho, a murderous masked vigilante who leaves the Jack of Diamonds behind with his victims as a literal calling-card. And as a whole the tale can be taken as a send-up of the kind of mindless action movies that Hollywood churns out like sausages.

But what makes “Seven Psychopaths” a lark isn’t any deeper meaning but the string of pleasures along its gruesome route to Billy’s long-for shoot-out. There are interpolated yarns about a Quaker father (Harry Dean Stanton) whose vendetta against his daughter’s killer never ends, and an erstwhile Viet Cong looking to avenge a massacre at his village, as well as more sentimental episodes between Hans and his dying wife. More frequent, though, is the banter among the leads, which takes typically loopy digressions that turn out to be integral elements of the central “plot.” And, of course, the equally frequent explosions of violence—ever more extreme—that give the humor a bitter aftertaste.

Still, all of McDonagh’s verbal dexterity wouldn’t mean much if the cast weren’t so fully attuned to it. Unlike in “Bruges,” Farrell is the supposed oasis of sanity here, frazzled but still assembling material for his planned opus from the events he’s trapped in. That leaves Rockwell to take the berserk path, alternating a goofy smile with an equally goofy intensity. As for Walken, all he needs to do is to shamble along in his usual offbeat fashion, giving each line a quirky reading all his own. Harrelson’s icy madness is more conventional—we’ve seen this act before—but in truth the material McDonagh provides for him is among the screenplay’s less interesting elements. Waits, on the other hand, gives an amusing spin to his rabbit-obsessed hit-man, and Stanton brings his inimitable personality to the determined Quaker—and nothing more is needed.

Cinematographer Ben Davis adds to the cartoonish quality of the picture with very brightly colored images, as does Carter Burwell’s score, which blends the silly and the serious. All the technical credits are fine, with the production design (David Wasco), art direction (John Dexter), sets (Sandy Wasco) and costumes (Karen Patch) contributing to a flashy overall look.

Ultimately “Seven Psychopaths” doesn’t amount to much, but like tasty junk food it goes down easily, even if the nutrition is nil.