R-rested Development: The vulgar immaturity of Step Brothers’ main characters extends to the filmmaking.

Brennan Huff (Will Ferrell) and Dale Doback (John C. Reilly) are 40-year-old losers, unemployed and still living at home with their respective single parents (Mary Steenburgen and Richard Jenkins) when those parents meet and decide to get married. But Brennan and Dale are more than 40-year-old losers; they’re 40-year-olds trapped in their 12-year-old emotional development. Brennan struggles not to burst into tears when Dale insults him, and Dale avoids the nearby playground with its bullying schoolchildren. They’re equally fascinated with Chewbacca, night-vision goggles and Hustler magazines. And, of course, there is nothing funnier to them than naughty words as punch lines.

Ferrell and his frequent writing partner Adam McKay teamed up on the surreally goofy hit comedies Anchorman and Talladega Nights, both of which rolled into theaters with PG-13 ratings. But something happened between Talladega Nights and Step Brothers– specifically, an infamous viral video short called The Landlord. Starring a profanity-spewing toddler, The Landlord became a phenomenon– and apparently it convinced Ferrell and McKay that if you’re making a movie about vulgar and immature characters, the movie itself should also be as vulgar and immature as possible.

Hold your dismissals of these as the ramblings of a prudish crank; sometimes, as in the writing/directing works of Judd Apatow (a producer here), low humor can be ridiculously entertaining. But when it works, it’s because there’s something else going on besides the impulse to offend. There’s a difference between using your f-bombs as seasoning, and offering them up as the meal itself.

None of which means it’s completely worthless. Ferrell and McKay do know how to carve laughs out of bizarre non-sequiturs, like the mutterings and random destructiveness of Brennan and Dale’s joint evening of sleepwalking. There’s a priceless bit involving Brennan’s over-achieving younger brother Derek (Adam Scott) leading his family in a four-part harmonized version of “Sweet Child o’ Mine,” and another musical number that improbably juxtaposes an operatic aria with the background rap lyric “boats and ho’s.” And yes, it’s hard to deny the simple visual kick of the film’s two paunchy stars wandering around in jockey shorts and vintage Pablo Cruise and Judds T-shirts.

Anchorman and Talladega Nights also functioned more as collections of individual gags than cohesive movies, but in those cases the gags actually had punch lines. Step Brothers, in nearly every case, replaces an actual creative idea with variations on the magic four-letter word– or, when they’re feeling particularly adventurous, with something like “camel dicks” instead. And it’s not simply a case of the movie’s socially retarded central pair letting ’em fly because it’s appropriate for the characters’ personalities; it’s considered just as valid to have sexagenarian professionals Steenburgen and Jenkins blast the epithet as well. After 90 minutes, the need to attach a bodily function, a body part, a sexual reference or a profanity to nearly every single line of spoken dialogue simply comes off as tiresome and dizzyingly lacking in imagination.

STEP BROTHERS (2) Directed by Adam McKay. Starring Will Ferrell, John C. Reilly and Richard Jenkins. R, 97 min. • At Century Cinemas Del Monte Center. Maya Cinemas, Northridge Cinemas.