Quaint and crass get together — or would that be “bump uglies’’? — with awkward, thoroughly flat results in “The Big Wedding,’’ an ensemble comedy with a tonal cluelessness as surprising as the name cast that signed on for it anyway.
Adapted by writer-director Justin Zackham from the French import “Mon frère se marie,’’ the film stars Robert De Niro and Diane Keaton as Don and Ellie, a long-divorced couple whose reunion is, well, a farce. As everyone gathers for the marriage of the couple’s adopted son, Alejandro (Ben Barnes, “The Chronicles of Narnia’’), they encounter a snag. Alejandro’s biological mom (Patricia Rae) is on the way from Colombia, and it turns out that she has a contrived, pathologically traditional objection to divorce. What to do, what to do? Don and Ellie reluctantly decide to pretend they’re still married — an idea that, go figure, doesn’t sit well with Bebe (Susan Sarandon), the free spirit whom lusty Don left Ellie for years ago.
This being a “big’’ wedding, of course, the story spoons out plenty more rom-com fluff. Topher Grace plays adoptive brother Jared, a 29-year-old doctor who, for no discernible reason, is still a virgin, but who’s ready to lose it with Alejandro’s flirty biological sister (Ana Ayora). Katherine Heigl makes the requisite showing as adoptive sister Lyla, bravely smiling through fertility issues. Alejandro, fiancee Missy (Amanda Seyfried), and the fam have to deal with Father Moinighan (Robin Williams, refreshingly playing this as black comedy), a priest who dourly paints Pre-Cana couples as sinners on the fast track to hell. And what do you know, maybe Ellie and Don still have a fleeting thing for each other after all.
An opening in-flagrante moment with De Niro, Sarandon, and Keaton makes us cringe, but it’s novel enough that we’re receptive to more: OK, they’re out to take risks — maybe this will all click better as it goes, settle into an “I’ll have what she’s having’’ groove.
Instead, Zackham (writer of “The Bucket List’’) just keeps the unfortunate “ick’’ elicitors coming, seemingly convinced this passes for riotously grown-up wit. The group sits down for a fancy restaurant dinner, and promptly starts chatting — doesn’t every family? — about Ellie’s recent nine-hour tantric orgasm. (Hence Don’s rekindled interest, not coincidentally.) And at church, Ellie grouses that she’ll take confession when pigs fly out of her . . . oh, suffice it to say that even pervy old Don starts grumbling about images he could do without. Us, too.