Tawdry and overheated, featuring arguably the blowsiest cinematic catfight since Whatever Happened to Baby Jane?, this year’s surprise hit thriller Obsessed is no one’s idea of a respectable movie. The story follows an investment banker (Idris Elba) whose marriage to his former secretary (Beyoncé Knowles) is threatened by a psychotic office temp (Ali Larter). Mashing up elements of Fatal Attraction, Disclosure, and even Jungle Fever, the movie turns more hysterical and daffy as it goes along. By the climax, Knowles is forced to deliver lines like “You think you’re crazy? I’ll show you crazy. Just try me, bitch,” while Larter finds herself literally swinging from the chandelier.

Yet if Obsessed wins no points for elegance, it certainly shows us a cheesy good time. And its considerable success at the box office (it debuted at number one and grossed more than $65 million domestically) reveals something else unexpected: Houston-born Knowles, who rose to international celebrity as the lead singer of Destiny’s Child before moving on to a platinum-selling solo career, has fashioned herself into a movie star. She has appeared in only six films in the past seven years—one an instantly forgotten trifle (The Fighting Temptations), another a certifiable dog (The Pink Panther). But along the way she has made intelligent choices, shifting easily between artistic-minded projects (Dreamgirls, Cadillac Records) and commercial ones (Austin Powers in Goldmember) and performing with a modesty you might not expect from someone said to be a diva. Obsessed, which was marketed partly on the strength of her brand, brought her slow-burning film career to a boil. More than any crossover since Bette Midler, she seems to understand how to cultivate a screen image that can last for years. She might even have the chops to become a major actress.

The path from musical to cinematic icon is, of course, a notoriously rocky one that has produced more than a few female casualties over the years. Sometimes pop stars choose to play unintentionally campy versions of themselves (Mariah Carey in Glitter). More often, they prove to have truly dreadful taste in material (everything Madonna made after Desperately Seeking Susan and everything Jessica Simpson has ever made). In the case of Jennifer Lopez, early promise in Selena and Out of Sight devolved into woebegone vanity projects like Bordertown and El Cantante. (Male musicians seem to have an easier time of all this, perhaps because so many of them—including Willie Nelson, Mos Def, and Justin Timberlake—are content to play supporting characters and leave the heavy lifting to the “real” actors.) But right from her splendid debut, in Goldmember, playing Austin Powers’s tough-talking, pistol-packing, blaxploitation-inspired sidekick, Foxxy Cleopatra, Knowles seemed to grasp what many others have not: As an inexperienced actress, you need to push yourself out of your comfort zone, but not so far that you make a fool of yourself. Sporting a giant Afro and a series of gorgeously form-fitting leather and lamé costumes, she glides through her scenes with an infectious easy-does-it grin, putting a honey-coated spin on decidedly dopey dialogue (“You have the right to remain sexy, sugar”). The part is so small and goofy that her technical limitations hardly seem to matter.

That willingness to blend into an ensemble and never bite off more than she can chew has served Knowles very well. Despite the accusations that have been leveled at her over the years—that she and her father, Matthew Knowles, elbowed out the ablest members of Destiny’s Child and that her ego eventually caused the group to disband—she never tries to make herself bigger on-screen than the material. Take another look at her underappreciated work in Dreamgirls, in which she plays the Diana Ross-inspired Deena Jones, a beautiful but only marginally talented musician who is chosen to replace her less glamorous friend (Jennifer Hudson) as the lead singer of a sixties girl group. In her scenes opposite Hudson, Knowles graciously steps aside and allows her fellow crossover to sing her right off the screen. But she puts forth a convincing portrait of youthful optimism curdling into middle-aged disappointment, and her heartfelt rendition of “Listen,” a song she co-wrote, lends feeling and cohesion to a narrative that in its original stage incarnation meanders tediously in the second half.

Even more interesting is her turn as R&B great Etta James in Cadillac Records, an ambitious drama that chronicles the rise and fall of Chess Records founder Leonard Chess (Adrien Brody). Knowles, who served as one of the film’s executive producers, emerges at the one-hour mark wearing a blond wig and a merciless glare and brings the somnolent proceedings back to life. Granted, this is hardly the most deeply lived-in study of an out-of-control heroin addict ever seen, but Knowles sings James’ standards, including “At Last” and “I’d Rather Go Blind,” with an impressive mixture of ferocity and tenderness. She even manages to achieve one thing that no other actress before has achieved: She generates occasional sparks of erotic heat opposite the clammy, perpetually remote Brody.

In her first five films, Knowles played singers—an obvious choice that was starting to become a crutch. As long as you have a soulful pop ballad to stop the show, you never have to dig too deep into the psyches of your characters. (There are also only so many projects in which the main characters periodically burst into song. Just ask Diana Ross, whose film career never survived beyond Lady Sings the Blues and The Wiz.) In Obsessed, which Knowles also executive-produced, the woman she’s playing is paper-thin and insultingly retrograde—a bitchier version of Anne Archer in Fatal Attraction. Opposite a steady smolderer like Elba and a canny scene-stealer like Larter, her performance is at best serviceable. But for the first time she’s working in realist mode, playing a figure you theoretically might encounter at the grocery store (assuming you live in a town where the only cop is played by Christine Lahti looking as if she took a wrong turn on her way to a guest-starring spot on CSI). She seems to realize that if you want to get better, you can’t keep using glittery gold eye shadow as a buffer between you and the audience.

So how does Knowles take it to the next level? She should spend more time with the kind of serious acting coach who could help her sharpen her occasionally amateurish-sounding line readings and might encourage her to bring her own life experiences to bear on the people she’s playing. After that, it’s time for her to reclaim a bit of her inner diva; no film actress became great by ceding the floor repeatedly to a much flashier co-star. Finally comes the trickiest task: finding that meaty role that will make us look upon her—à la Halle Berry in Monster’s Ball, Charlize Theron in Monster, or Anne Hathaway in Rachel Getting Married—as never before. Having proved she’s a draw at the box office and having shown a knack for comedy, drama, thriller, and musical, Beyoncé Knowles now needs to risk making a fool of herself.

Single Laddies: Good spoof, great sport.

If you’re still not convinced of Beyoncé Knowles’s acting prowess, take a look at one of the highlights of the 2008-09 season of Saturday Night Live: a bizarro spoof of the music video for her chart-topping song “Single Ladies (Put a Ring on It).” (You can find it on a number of content-sharing sites, including Digg.com.) Paul Rudd plays the video’s director, who has cast his three stepsons—played by Justin Timberlake, Andy Samberg, and Bobby Moynihan in skintight black leotards and high heels—as Knowles’s backup dancers. But each time the music starts, they begin gyrating uncontrollably, causing her to repeatedly yell, “Cut!” Knowles’s ability to keep a straight face as Rudd delivers lines like “Beyoncé, that was bee-rilliant” is impressive. And just like Timberlake, she seems to realize that there’s something very endearing about a pop star turned thespian willing to poke fun at her own celebrity.