rated R

During the last decade, Paul Rudd has gone from a supporting player to a major co-star. Once “that guy,” Rudd is now ubiquitous, and the progression has been so smooth that it’s hard to remember a time when Rudd wasn’t showing up in a major comedy every year. His latest feature offers another transition for him, this time from co-star to lead. As star vehicles go, “Our Idiot Brother” is perfect for Rudd and proves that he can carry a movie on his own. But while Rudd has graduated to leading-man status, the uneven script leaves the rest of the cast behind.

Clad in Crocs, old shorts, a ratty T-shirt, and a beatific smile, Rudd stars as Ned, a dopey hippie who works on a farm and hangs out with a golden retriever named Willie Nelson. While selling organic veggies at the local farmers’ market, Ned is tricked into selling weed to a uniformed cop. After a quick stint in jail, Ned returns home to find his life upended. His ex-girlfriend Janet (Kathryn Hahn) has kicked him off the farm and refuses to give him custody of Willie Nelson. Without a home and without his dog, Ned turns to his family, specifically, his three sisters, Miranda (Elizabeth Banks), Nat (Zooey Deschanel), and Liz (Emily Mortimer), all of whom have distanced themselves from their lackadaisical brother.

The sisters are all busy Brooklynites, wrapped up in complicated relationships, jobs, and social lives. Miranda is trying to get a story published in Vanity Fair; Nat, an amateur stand-up comic, is a commitment-phobe who’s cheating on her girlfriend Cindy (Rashida Jones), and Liz is a helicopter mom oblivious to her husband’s (Steve Coogan) infidelities. Ned, of course, upends their lives as well and exposes the hypocrisies and contradictions they’ve all ignored for so many years.

Ned’s a fool, but a holy one, spreading sweetness and light to all the squares he encounters. He’s a close cousin to The Dude in “The Big Lebowski,” but in search of his dog instead of a new rug. It’s a great role for Rudd, whose characters are usually put-upon guys fueled by snark and sarcasm. In “Our Idiot Brother,” he’s a reverse straight-man, befuddled but calm, and the best moments come from his non-reactions to the familial insanity surrounding him.

Unfortunately, that family is the weakest part of “Our Idiot Brother.” The sisters are flat, one-note characters, as joyless as Ned is joyful. That they all need Ned’s help to get their lives on track seems disingenuous, and the screenplay, by Evgenia Peretz and David Schisgall, makes them unlikable enough that it’s tempting to root for Ned to spurn them completely. Ned’s kind of dopey, but he’s not a bad guy, and the movie might easily have been called “My Mean Sisters.” Banks, Deschanel and Mortimer have great chemistry with each other but never sell the sisterly relationship.

The supporting cast, though, is great. Adam Scott, Rashida Jones, and Steve Coogan all give their small roles the same kind of verve that Rudd used to give when he was in the periphery of a film.

For all its faults, “Our Idiot Brother” is one of the more pleasant comedies of the last year. It’s largely because Rudd’s charm can make even the most dismal flick watchable. It’s also because Peretz, Schisgall and director Jesse Peretz (Evgenia’s brother) have made a comedy that relies on its characters and not elaborately-constructed gross-out scenarios or aggressive awkwardness. Nobody gets pooped on or violently assaulted, which makes “Our Idiot Brother” seem like its protagonist, a smiling idealist swimming in a sea of cynical jerks. For that reason alone, it’s fresh. That it propels Rudd up another step on the comedy ladder is a nice bonus.