NEW YORK — The song is only about five minutes long. But it is the purest example of a showstopper. Sung poorly, it is a rafter-shaking cliche. Sung well, it can be the vocal equivalent of the Rapture — transcendent and soul-stirring. And it has the proven ability to bestow a lifetime of fame.

Jennifer Holliday sang “And I Am Telling You I’m Not Going” when the musical “Dreamgirls” opened on Broadway on Dec. 20, 1981. The show tells the professional and personal story of a 1960s girl group modeled after the Supremes. Holliday originated the role of Effie White, the emotional center of the tale, the one whose heart is broken. “And I Am Telling You” marks the show’s turning point, the moment when Effie learns she has been pushed out of the group and betrayed by the man she loves.

Holliday threw herself into that song, won a Tony Award and assured her place in Broadway history.

Twenty-five years later, “Dreamgirls” has become a film, which opens everywhere Christmas Day. Jennifer Hudson, a former “American Idol” contestant, plays Effie. And thanks to a five-minute song, her life is in the midst of exhilarating, unnerving change.

The film stars Eddie Murphy, Jamie Foxx, Beyonce Knowles and a host of other actors with a wealth of experience. But Hudson has custody of the story’s iconic moment. When she sings, she is both sassy and pain-stricken, confident and vulnerable. Audiences have erupted in applause and cheers. She’s been nominated for a Golden Globe. Prognosticators see an Oscar in her future.

What is it like when fame shows up so suddenly and with such fanfare? What does it feel like to have Oprah Winfrey describe your singing prowess as akin to a religious experience? What do all the accolades sound like when two years ago you didn’t have an agent or a manager — when all you had were your prayers? This moment, she told Winfrey when the cast appeared on her show, feels “like God’s favor.”

“That’s the best way I could try to describe it,” Hudson says later. “There’s a time for everything. It’s my season. It’s the same voice and the same girl. It’s nothing but the favor of God.”

Hudson, 25, grew up in Chicago, the youngest of three siblings. She worked as a singer on a Disney cruise ship where her co-stars kept up a running dialogue about Broadway musicals and told Hudson that she was destined for the Great White Way. In 2004, she was a contestant on “American Idol” and was told by judge Simon Cowell, on national television: “I think you’re out of your depth in this competition.” Her jaw dropped in astonishment. (Cowell later ate crow on “Oprah.”) The public voted Hudson off. Her friend Fantasia Barrino won.

She worked on her music, and kept busy performing in small venues. And then one day a casting agent in Los Angeles called and asked her to audition for “Dreamgirls.”

Director Bill Condon tested 782 other women for the role. Each actress had to perform the same bit — the “what about me” scene — but only when the group was narrowed to a handful did each sing “And I Am Telling You,” live with piano accompaniment. The choice, Condon says, was obvious. “Jennifer, not having done this before, was kind of fearless.”

For the role, Hudson used personal moments of rejection and disappointment. Her experience on “American Idol” informed the role, in part because it was so fresh in her mind. She kept a makeshift diary of Effie’s relationships and aspirations. She took Effie home with her, eating the high-calorie food that Effie ate, listening to the 1960s music that Effie might have heard.

“I felt like I was Effie. Jennifer was nowhere in sight. Everything had to change,” she says. “When I turned on the TV to the video channel, I wanted to hear the Supremes.”