The photomontage that opens the anti-romantic comedy “The Break-Up” shows Jennifer Aniston and Vince Vaughn canoodling before the fight that sets up the action of the film. As snapshots of the two flash onscreen, it’s hard not to compare them. Ms. Aniston seems positively luminous next to Mr. Vaughn, who has the look of an average man who has snagged a girl outside his means. But once they open their mouths, all that changes.

Mr. Vaughn may not look like a traditional heartthrob, but his fast paced insult improv has won him a devoted following. This film seems an attempt to turn his aggressively sexual sidekick character into more of a leading man. Ms. Aniston seems game enough to join him in this task, but she’s often left looking like she can’t keep up.

The film has Mr. Vaughn playing Gary, a Chicago tour bus guide who woos Ms. Aniston’s art dealer, Brooke. And while the trailer of the film tells audiences to “pick a side,” it’s sometimes hard to sympathize with either of them.

Their infamous break up is brought on by an inane fight that doesn’t seem far from what the rest of their relationship might have been like. From there, each of them tries to instigate the other into getting back together. An odd strategy, that has mostly predictable results.

Mr. Vaughn’s Gary is an overgrown child obsessed with video games and his couch. Ms. Aniston, an art dealer with a desire to go to the ballet but no means of getting there without her immature boyfriend, just seems like a glutton for punishment. One of Gary’s buddies tries to console him after the break up by saying, “You shouldn’t even feel bad. She should have expected it.” And it’s hard to disagree.

The filmmakers have done their best to accommodate Mr.Vaughn by casting him as a quick-witted tour guide and letting him run loose. It seems as though much of the dialogue is ad-libbed, which has many benefits, but also the unfortunate side effect of leaving Ms. Aniston behind.

In most of his films – “Swingers,” “Old School,” “Wedding Crashers” – Mr. Vaughn specializes in playing the hypersexed, overbearing buddy. His performances have helped make many of those films successful, but he needs a good straight man to play against. The men who play his pals here – Jon Favreau, Jason Bateman, Cole Hauser – may have great chemistry with Mr. Vaughn, but the female lead does not.

Luckily this is a break up movie, so that might not be so important. But break ups often highlight people’s worst character traits. This film trades in the ridiculous and selfish antics that ensue with the decline of a relationship – to the detriment of the main characters’ likeability.

Despite her looks and her seeming attempts to be a good girlfriend, Brooke’s antics often make her come off as an irritating nag. And like Ms. Aniston, who may have the makings of a movie star on paper, Brooke has trouble with her delivery.

Ms. Aniston – and her fantastic hair – has had much less success on the big screen than she did with “Friends.” Though she has had some cinematic successes and studios have seemed more than willing to test her out at the box office recently, she is still plagued by an odd blankness onscreen.

She can play underachieving stoners with disturbing accuracy (see “The Good Girl” and “Friends With Money), but Ms. Aniston often seems adrift outside her comfort zone.

And while “The Break-Up” appears to be outside her range, it’s a bit too far inside Mr. Vaughn’s to work. Mr. Vaughn excels as a childish and persistent egotist, but his scenes with Mr. Favreau often make the film seem like a “Swingers” redux for audience goers who might want to know how those characters might deal with an adult relationship.

As the two leads go to their respective corners and initiate battle, the only thing the film seems to be making is an argument against premarital cohabitation. The pairing of a tour guide with an aesthete seems like a bad idea that they could have easily rectified if they weren’t bonded through real estate. The two go back and forth between wanting to get back together and hurt each other, but the only true casualty of their relationship seems to be their fleeting grasp on a lovely sun drenched condo.

mkeane@nysun.com