In case anyone in the audience isn’t sure when to cackle, coo, snicker or sigh, the makers of “Old Dogs” have provided a handy on-screen prompt.

It’s an old dog, shown in close-up, reacting with little grunts of canine confusion or curiosity over the antics of Robin Williams and John Travolta in this dead stray of a family comedy.

Director Walt Becker cuts away to the pooch, the aging pet of Travolta’s character, again and again. So often, in fact, that maybe Sebastian, the 9-year-old dog playing the creaky old pet, should have shared top billing with Travolta and Williams in this rubbish about middle-aged buddies caring for young twins one of them never knew he fathered.

At some point, you half expect to hear the dog’s thoughts, like the yammering pets in “Look Who’s Talking Now,” the last in Travolta’s talking-baby series (a movie franchise that seems like Preston Sturges compared with this).

Whatever the old dog might want to say, it’s bound to have been more interesting than anything uttered by the two-legged creatures in “Old Dogs.”

Becker, who scored a hit working with Travolta on 2007’s dumb but inoffensive “Wild Hogs,” takes things down a notch or two for “Old Dogs.”

Travolta plays swinging bachelor Charlie to Williams’ mopey Dan, pals since childhood who run a successful sports-marketing agency.

The movie opens with a flurry of ham-fisted flashbacks as Charlie inexplicably starts a crucial business meeting by chronicling Dan’s whirlwind marriage to Vicki (Kelly Preston, Travolta’s real-life wife) seven years earlier.

Dan and Vicki split immediately, but the brief marriage resulted in twins (Conner Rayburn and Ella Bleu Travolta, Preston and Travolta’s daughter). With Charlie and Dan caught up in negotiations for the biggest deal of their careers, Vicki arrives needing to stash the kids with their dad for two weeks while she’s off doing jail time for a political protest.

Will the parentally inept Dan prove he can be a dad to his twins? Will the kids’ shenanigans foul up Charlie and Dan’s big deal? Will their friendship survive the strain, will business or family win out? Will Sebastian ever speak his mind?

We all know the answers before the movie starts. The fun is supposed to be in getting there, but “Old Dogs” is no fun whatsoever, just a collection of stale kibble.

Working from a screenplay by David Diamond and David Weissman, Becker offers bad gags about doggie incontinence, golf balls to the groin and repetitive jokes about people mistaking Travolta and Williams for grandparents.

The physical humor is crude and often cruel, a hand model’s precious digits mashed by a car trunk, a Frisbee football game that gives way to mean-spirited elbows and crushing tackles.

A family affair for Travolta, the movie also serves up a couple of his siblings among a group of incredibly annoying singing waiters and waitresses (yeah, singing waiters are supposed to be annoying; doesn’t make them funny, though).

Emitting falsetto shrieks much of the time, Seth Green makes a nuisance of himself as Dan and Charlie’s protege, while Lori Loughlin just simpers along as a translator that wolfish Charlie hits on.

Matt Dillon, Ann-Margret and the late Bernie Mac turn up in succession as “Old Dogs” stumbles from one clumsy anecdotal sketch to the next.

Luis Guzman and Dax Shepard appear in terribly unfunny bits as feuding child-proofers, though luckily for them, both actors are unbilled, so they can avoid their share of the blame for this mess.

Travolta and Williams truly put some energy into their characters, but there’s nothing there for them to play. “Old Dogs” is so empty-headed, all they can do is put on goofy, sometimes creepy, faces and try to make observations about their aging bodies sound amusing.

We understand that actors want to keep busy, they want to work with people they admire, they have to do commercial projects to buy the Hollywood goodwill to film those small, artsy things they really want to make. And of course, they do want to make lots of money.

But come on, guys. Did the family dog eat all the other big studio scripts you had piled by your bedsides, leaving only this poo to thumb through?

If Sebastian could talk, even he might say, “I need a better agent.”

Movie Review

“Old Dogs”

One and a half stars

Rated:PG

Robin Williams and John Travolta star in a predictable story of old friends who unexpectedly find themselves taking care of twins. With Kelly Preston and Ella Bleu Travolta, left. Directed by Walt Becker. 88 minutes.