Katherine Heigl may have been the sleeper hit of 2007. The buxom blonde managed to position herself as a model young spinster by helping Hollywood to avoid the unpleasantness of having to watch an unattractive woman be lonely and unhappy.

In both Judd Apatow’s “Knocked Up” and the ongoing NBC series “Grey’s Anatomy,” Ms. Heigl disappears into successful ensemble casts. But in Anne Fletcher’s “27 Dresses,” which opens in sneak previews today in New York, she takes top billing while retaining her willingness to share the screen with her co-stars. Screenwriter Aline Brosh McKenna’s cynical approach to weddings cuts into the film’s happy ending, but reliable performances and a popular topic will likely get “27 Dresses” bank at the box office.

Nearly every woman older than 20 has at least one terrible dress in her closet and residual feelings of resentment toward a close friend stemming from bridesmaid duties. “27 Dresses” posits what might happen if the consummate bridesmaid were called upon to plan a wedding for the man of her dreams — who happens to be marrying her sister.

Ms. Heigl plays Jane, a woman so in love with her boss, George (Edward Burns), that she has remained his assistant long past the point where the job helps her career. An environmentalist entrepreneur who loves dogs, George has about as much personality as Al Gore napping. The supposed niceness of his character is hampered by the fact that he has permitted a woman he cares about to continue to fetch him coffee every morning for years. Of course, Jane’s absurdly cinematic starter apartment leaves open the possibility that she is the best-paid assistant in New York City.

When Jane’s sister Tess (Malin Akerman) comes to town, she immediately wraps George around her finger. Too nice to mention her own feelings for him, Jane takes on the role of maid of honor for Tess and George’s wedding. Before long, her 28th donning of a bridesmaid’s dress draws the attention of wedding columnist Malcolm Forbes (James Marsden). Trying to “break out” of the wedding beat with this story, Malcolm, whose real name is Kevin, starts to woo Jane in the hope of landing his exposé. In the process of his undercover assignment, you will not be shocked to learn, he falls for her.

Like the wedding section of many newspapers, “27 Dresses” often relies more on the female obsession with all things bridal than on a strong story. To name just a few glitches: Despite the fact that she’s planned 27 weddings, Jane’s only friend is an alcoholic accountant (Judy Greer) who works in her office; a lonely, single woman in New York spurning the advances of an attractive man who turns out to be her favorite writer is just wishful fiction; Jane’s relationship with her sister is poorly conceived, and the man they both love has all the charm of dry wood.

Fortunately, aside from Mr. Burns’s narcolepsy in the role of George, the rest of the supporting cast ably keeps the film moving along. Ms. Heigl and Ms. Akerman often manage to speed past holes in their characters’ personalities, while Ms. Greer relishes the departure from her normal geeky sad-girl roles. And Mr. Marsden, whose polished looks have often relegated him to two-dimensional characters, brings most of the life to the film.

Like Ms. Heigl, Mr. Marsden seems to be battling against his looks in various roles. The actor has been consigned to pretty-boy roles (“X-Men”), noble-but-boring-guy roles (“The Notebook”), and smiling bubble-gum roles (“Hairspray”). He deftly lampooned his standard character with his turn as Prince Edward in Disney’s recent “Enchanted,” but as Kevin in “27 Dresses,” he demonstrates his able potential as a leading man.

The “always a bridesmaid” concept of the film will likely lure women to theaters and prove Ms. Heigl’s worth as a bankable star. The film misses its mark as often as it hits it, but it’s close enough for wedding work. The lead actors have palpable chemistry, and in romantic comedies, that’s often all you need.