Near the end of Monster-in-Law, Jennifer Lopez and Jane Fonda engage in an epic slap-fight--and the audience at last gets what it came for.

Oh sure, we can pretend it’s not so. After all, maybe the American zeitgeist just finds clashes between prospective in-laws hilarious at the moment, what with Meet the Fockers, Guess Who and this film showing up in theaters within 12 months of each other. But while we could be certain some slapstick hilarity would ensue when fathers fumed over the guys preparing to take away their little girls, no one really expected the combatants to roll around on the floor and tear at each other’s hair. From Monster-in-Law, we want women behaving badly--and if the participants are two women whose public lives have given plenty of people over the years reason to want to see them take one in the chops, so much the better.

I know there are people who find the dueling humiliations comedy of movies like Monster-in-Law hilarious. I’ve never actually met any of them, but I hear them all around me during press screenings. But doesn’t it help if the central conflict doesn’t feel sanitized for the actors’ protection? Or if the people doing most of this theoretically funny stuff have, say, an ounce of comic timing?

That’s an ounce more than you get from Jennifer Lopez, who here plays a free spirit called Charlie--a “free spirit” generally being identifiable by the kooky name he or she uses. Something about her kooky free-spiritedness attracts hunky surgeon Kevin Fields (Michael Vartan), who soon wants to introduce the new love of his life to mom. Unfortunately for Charlie, mom is Viola Fields (Fonda), a diva of a TV celebrity interviewer who has just found out that she’s being replaced by a prettier, younger face. Left fragile by an on-air nervous breakdown, Viola isn’t about to have any of her precious boys pairing off with a “temp.” And the fur proceeds to fly.

At least a little bit of fur, on one side. Fonda--the veteran returning to film after years as Mrs. Ted Turner and a national mea culpa tour--proves fairly game, shrieking and mugging furiously as she tries to pump some energy into the film by getting into the spirit of entitled fabulousness. It’s a role that could have been played just as easily by the likes of Shirley MacLaine--and, in all likelihood, better--but Fonda shows little career rust as she plots and preens.

Her competitor, on the other hand, is no competition at all. Some of the humor ostensibly comes from watching Charlie--whom we know is super-nice because she walks dogs and is kind to baristas--sink to the level of her competition, only director Robert Luketic (Legally Blonde) isn’t about to let Lopez look genuinely nasty. Stars rarely are willing to look that way, so we get the character equivalent of Barbra Streisand always getting the most flattering light possible on her fingernails. The shame of it is that Lopez’s rather limited acting range seems best suited to letting her go over the top--and she almost never gets the chance.

The real problem with Monster-in-Law becomes its unwillingness to bare its teeth. Those who come to see a knock-down drag-out are instead going to see a middleweight and a lightweight dancing around the ring, poking at each other tentatively before getting into the occasional clinch. What little actual spark there is in the film is left to Viola’s personal assistant and confidant Ruby--who, in a risky break from diva tradition, is a black woman (Wanda Sykes) rather than a gay man. (Note: J.Lo kindly makes up for this glaring omission by having her character’s guy pal play for the other team). Sykes could break up a room just standing still and staring, and I’m convinced that every one of her lines is ten times funnier coming out of her mouth than it was on the page.

That’s because Sykes is a performer with an edge, whereas the rest of Monster-in-Law is calculated not to make anyone appear too unlikable. By the time Lopez and Fonda finally get down to some serious cheek-smackin’, it’s so long overdue that you may have forgotten why you came to the movie in the first place. How much entertainment value can there be in watching women behaving blandly?