Writer-director Lynn Shelton came to the fore with her 2009 bro-mantic comedy Humpday about two straight male friends, who drunkenly agree to have sex with each other on film for an art project.

The acutely observed and provocative tale of brotherly bonding in extremis laid the groundwork for Shelton's subsequent pithy surveys of the sexes in Your Sister's Sister and yesteryear's Touchy Feely.

Say When, her sixth feature, marks a concerted shift away from her low-budget indie roots towards glossier mainstream fare.

It's also her first film based on someone else's script - a gamble that doesn't pay off, sadly.

Screenwriter Andrea Seigel hinges her freewheeling narrative on a beautiful 28-year-old, who has no direction in her life and is content to wander aimlessly from one uneventful day to the next in the company of friends from school, who all seem to know where they are heading.

She's the least likeable and sympathetic character in the film, which poses an unsurmountable hurdle for Shelton, who has to convince us to care deeply about her lackadaisical and deceitful heroine.

The slacker in question is Megan (Keira Knightley), who has an impossibly patient and understanding boyfriend called Anthony (Mark Webber), and works for her father Ed (Jeff Garlin) by twirling a sign at the roadside.

At the wedding of best friend Allison (Ellie Kemper), Megan sidesteps a marriage proposal from Anthony and catches Ed in a clinch with a woman who certainly isn't her mother.

Shell-shocked by her vacuous world spinning off its axis, Megan heads to the grocery store for wedding reception supplies and ends up buying alcohol for disaffected 16-year-old Annika (Chloe Grace Moretz) and her chums.

Charmed by the teenager's carefree attitude to life, Megan tells Anthony that she is heading out of town for a week to attend a seminar but really heads to Annika's house and crashes on the floor of her underage friend's bedroom.

The youngster's father, a divorce lawyer called Craig (Sam Rockwell), eventually allows Megan to stay in the film's first stretch of credibility.

"Please don't let this decision become bad parenting on my part," he warns Megan and, almost inevitably, there's a spark between the divorcee and his new house guest.

Fittingly, Say When lacks the same sense of propulsion and purpose as the hapless heroine.

Even the title sounds wishy-washy.

Knightley, Moretz and Rockwell barely scratch beneath the surface of their unlikely co-habitants, while Webber is wasted in a thankless supporting role as the nice guy, who anchors himself to a woman that clearly doesn't appreciate his many charms.

Their broadly sketched characters trade glancing verbal blows as the script meanders towards its inevitable and lacklustre moment of reckoning.