Improvised narrative comedy is a dangerous tightrope, but in recent years the films of Christopher Guest (Best in Show, A Mighty Wind) and Larry David’s brutally funny HBO series “Curb Your Enthusiasm” have defied the odds and not merely maintained their balance but dazzled audiences with their agility. The British import Confetti, on the other hand, is a textbook example of what happens When Improv Falls Flat. Playwright and theatre director Debbie Isitt, in her third feature outing, gathered together an ensemble of actors familiar to Britons for their TV work and set them to the task of improvising the story of three couples competing in a magazine contest for the nation’s most original nuptials. The result feels as interminable as a wedding reception where you barely know the bride and groom.

The eponymous Confetti magazine will award a million-dollar mansion to one of these “zany” pairs of finalists: Matt (Martin Freeman) and Sam (Jessica Stevenson) plan to stage a full-fledged Hollywood-style musical, even though neither can dance or sing; avid athletes Josef (Stephen Mangan) and Isabelle (Meredith MacNeill) are going for a tennis-themed wedding, with dancing ball-boys; and Michael (Robert Webb) and Joanna (Olivia Colman) simply want to celebrate their love for each other—and nudism.

Of the three couples, only Matt and Sam are remotely engaging. Freeman, familiar from “The Office” (the U.K. original) and The Hitchhiker’s Guide to the Galaxy, brings his familiar hangdog presence to the movie, even if this is far from his shining hour, and his story is fleshed out with a supporting group that includes the formidable Alison Steadman (onetime leading lady of Mike Leigh films) as his future mother-in-law. The fiercely competitive Josef and Isabelle are one-note abrasive, and there’s a grotesque gag as Isabelle has disastrous plastic surgery to fix her gaping nostrils. The single joke about Michael and Joanna is that they like to be naked. (Oh, those naughty British!) Webb especially has more scenes of frontal nudity than any actor ever in an R-rated film (and that includes Ewan McGregor).

Oh yes, there’s another prominent couple in the movie: fey wedding planners Archibald (Vincent Franklin) and Gregory (Jason Watkins), who fully embody the expected stereotypes and set gay representation in movies back by another couple of years.

The only time the film truly comes alive is during the competition itself, as each couple marries in an elaborate ceremony staged in a huge ballroom. Because these scenes actually required some rehearsal and planning, there’s a glimmer of imagination here, especially in the rather charming, low-rent Busby Berkeley number Matt and Sam devise. But otherwise, the movie is a long slog through lots of unfunny, uninspired improv. A Wasted Talent production (we’re not making this up), Confetti is destined for the trash bin.