"Identity Thief" is a larcenous bit of funny business. It probably should be locked up for its crimes and misdemeanors against moviemaking.

But its stars, Jason Bateman and Melissa McCarthy, steal so many laughs from such improbable places that the bumps in this revenge/road trip farce can be mostly forgiven, though not forgotten.

Directed by Seth Gordon, the film has the same R-rated tenor of his relatively horrible "Horrible Bosses" and his really dreary "Four Christmases." More problematic, it has the same difficulty with the connective tissue — anything but the really funny stuff sags or is superfluous.

But the comedy itself is more finely tuned and the physical pranks are better, though that may just be Bateman and McCarthy doing their thing. The central issue is a wildly out-of-control script. Sorry, "Hangover Part II" Craig Mazin, but"Thief" is a sprawling mess packed with too many unrealized and uninteresting characters tagging along for the ride.

Many of the plot twists surrounding Sandy (Bateman) and Diana's (McCarthy) cross-country ride echo the road-trip madness of "Midnight Run" and "Planes, Trains and Automobiles." That would be fine if the film could keep pace with the classics. But too often "Identity Thief" lacks the tight calibration that kept you laughing and anticipating the next crazy curve in those much better comedies.

"Identity Thief's" mad dash begins in Winter Park, Fla., where Diana is about to run one of her credit card scams on the wrong guy. Sandy Bigelow Patterson is a decent sort, with an understanding wife (Amanda Peet), two really cute kids (real-life sisters Mary-Charles and Maggie Elizabeth Jones) and a baby on the way. He's a mid-tier accountant with a horrible boss (Jon Favreau). Though Sandy's basic M.O. is to take whatever life throws at him, he's frustrated enough to make a break with the firm's hotshot (John Cho).

Then, in a parody of one of our modern-day plagues, Diana calls with an identity "protection" offer and the walls start tumbling down. With his credit cards maxed out, criminal charges piling up and the police virtually no help, Sandy decides to take matters into his own hands. In short order, Diana's in handcuffs and the two "Sandys" are on the road to Colorado to clear the real one's name. Now the true fun of "Identity Thief" begins.

McCarthy is such a gem, with a rare ability to turn a thought into a one-woman show. Even small turns become priceless — the psycho mom in "This Is 40," a gastricly challenged member of the wedding in "Bridesmaids." The actress never fails to leave you wishing for more — she's the reason some moviegoers sit through the credits of any movie she's in, hoping for outtakes. "Identity Thief" is the biggest sandbox yet.

She takes over the entire playground in "Thief," throwing herself so literally into the action — scrambling out windows, running down freeways, colliding with oncoming cars — that it's a wonder she survived.

Though the actress often uses her plus size as a punch line in the movie, it serves even better for her character's emotional arc. Like John Candy in "Planes, Trains and Automobiles," Diana's the outsider looking with envy at everyone else's happily ever afters, and McCarthy is masterful at telegraphing that pain.

Against McCarthy's extremes, Bateman is the perfect foil. The actor has found a way to play the nice guy next-door without losing an edge that also makes him slightly lethal when you least expect it — except in "Up in the Air." As George Clooney's downsizing boss, he was nonstop lethal and terrific at it. But Bateman is best in roles like this where his everyman can fight back without losing his humanity.

As you might expect, the time Sandy and Diana spend on the road changes them in fundamental ways. Obstacles come in the form of a hit man and woman hired to take Diana out. A bounty hunter (Robert Patrick) is also hot on their trail because she's skipped bail. The only amusing one is Big Chuck (Eric Stonestreet), a big-hearted cowboy who falls for Diana during one ill-fated pit stop that involves a bar, too much alcohol and some really kinky sex.

There are sweet scenes with the fam — the Jones sisters never fail to charm — but the heart of this film is on the road with Bateman and McCarthy. If not for their brilliance, "Identity Thief" would be running on empty.

betsy.sharkey@latimes.com