“Something Borrowed” poses the question: What happens when you realize you’re in love with your best friend’s fiance? But the characters are either so ill-defined or unlikable, it’s hard to care whether they get out of this tricky situation with their emotions and relationships intact.

And that’s odd, and unfortunate, because “Something Borrowed” stars the ordinarily adorable Ginnifer Goodwin as a New York attorney who finds herself in that predicament.

Directed by Luke Greenfield (“The Girl Next Door”) and based on the novel by Emily Giffin, “Something Borrowed” introduces us to Goodwin’s character, Rachel, on the night of her 30th birthday. She’s quietly freaking out about the passage of time because she’s still hopelessly single. Meanwhile, her closest pal since childhood, the blonde party girl Darcy (Kate Hudson), is about to marry Dex (Colin Egglesfield), Rachel’s good friend from law school.

Rachel introduced the two of them six years ago and encouraged them to get together, even though she was secretly in love with Dex. (And Egglesfield, a former soap opera star in his first major film role, is traditionally handsome in a young-Tom-Cruise sort of way.) But after a few drinks at her surprise party, she and Dex end up sleeping together — and that inspires them to revisit feelings they’d both suppressed.

Clearly, they’re meant for each other, but each feels a responsibility toward Darcy — which makes no sense, because Darcy only feels a responsibility toward herself. The script from Jennie Snyder Urman renders her in such one-dimensional fashion, it’s hard to figure out what she does besides drink and shop. She may not even have a job.

So it’s baffling that Rachel and Dex, two intelligent, ostensibly decent-hearted people, have chosen to spend any time with her at all. It’s also obvious that Dex’s old-money parents would be happier with the sweet and proper Rachel rather than the flamboyant and obnoxious Darcy.

And so the majority of “Something Borrowed” features Rachel and Dex hemming and hawing over how to handle their burgeoning relationship, as the threat of the big day draws ever closer. Much of this takes place over dull weekends at Darcy’s Hamptons beach house with a cadre of supporting players.

John Krasinski co-stars as Rachel and Darcy’s childhood friend, Ethan, who mainly exists for cutaway reaction shots and sarcastic remarks (he does get a few amusing lines); Steve Howey plays Marcus, a gleeful womanizer whom Darcy insists Rachel should hook up with, which shows how little Darcy really knows or cares about Rachel; and Ashley Williams as the clingy Claire, with whom Ethan had a one-night stand he regrets. He is so desperate to avoid her, he pretends to be gay. That’s how horrible these people are.

Many melodramatic revelations come to light in the film’s third act, then they’re just as quickly resolved. But don’t feel daunted: You’ll probably have checked out mentally and emotionally long before any of this anyway.