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The Change-Up flips the switch, but not the script.

Some films are all high-concept premise. Others, like The Change-Up, manage to fall short even at that.

This frothy diversion plays out like a mash-up of Freaky Friday, The Odd Couple and Wedding Crashers, but without wit or insight or the bravery to confront the possibilities of its conceit.

As it opens, the movie quickly establishes the personalities and home lives of its two leads: Dave (Jason Bateman) is a married lawyer with three kids who approaches his work and home responsibilities with equal seriousness. His buddy since the third grade, Mitch (Ryan Reynolds), is an underachieving actor who lives the freewheeling life of a bachelor. Each has what the other envies. Or so they think.

The morning after the two simultaneously verbalize their desire to have the other’s life — during a late-night pee in a wishing fountain — they wake up to find they’ve switched bodies.

When they attempt to re-create the event the next morning, the fountain has conveniently been moved to parts unknown. Thus, the frantic search begins to find the magic display while not screwing up their lives in the meantime.

Here, the phrase “Don’t tell me how to live my life” gets chucked to the curb, because once the exchange happens, Mitch and Dave need each other’s considerable direction. That’s especially true of Bateman’s version of Mitch, who must somehow close a business deal that Dave’s career depends on.

Wedding Crashers director David Dobkin pulls the same duty here, working off an undercooked script by the writers of The Hangover. The result is a film that manages to elicit a few belly laughs, but is numbingly paint-by-numbers.

When it’s not busy being predictable, The Change-Up strains for an outrageousness that is ultimately fatiguing for its desperation. The opening baby-changing scene is unnecessarily gross, but at least it sets an accurate tone for the film. When Dave (as Mitch) discovers his acting gig is a “lorno” (light porno, we’re told), he’s inexplicably required to perform an unsavory act on a woman old enough to be his grandmother.

That scene is a good example of where The Change-Up’s script goes wrong. It embraces juvenile humor when it should be pushing past the limits of the characters’ comfort zones for great comedy. Instead, it gives us the sight of a ridiculously irresponsible Mitch (in Dave’s body) putting Dave’s kids in mortal danger.

The movie tries halfheartedly to add some meaningful ballast to the frivolity with running threads about Mitch’s strained relationship with his father (Alan Arkin, here for a payday) and Dave’s neglected wife (Leslie Mann). But the effort is a waste, because the script refuses to explore the very issues it expects will have you reaching for a tissue. As a result, the emotions, though well-played, feel unearned.

The Change-Up consistently avoids going deep into every subject it intentionally or accidentally brings up, including the nature of personality. When Mitch and Dave return to their own bodies, the relationship Dave (as Mitch) started with his assistant Amanda (Olivia Wilde) doesn’t miss a beat, even though the real Mitch possesses none of the shyness that she found such a turn-on.

Despite the film’s many flaws, Bateman and Reynolds are a pleasure to watch playing dual roles that are essentially polar opposites. And therein lies the appeal of The Change-Up.

Here’s the kicker about the film’s gimmick: It’s completely unnecessary. Nothing happens because of the switch that a smarter comedy couldn’t have revealed to its protagonists in a more believable way. As it is, Mitch and Dave never share the one scene that the film practically cries out for: discussing what they’ve learned about each others’ lives.

There’s a good comedy of mismatched buddies to be made out of Bateman and Reynolds playing these characters. It just isn’t The Change-Up.