Lupita Nyong’o was giving a surprise birthday party for her best friend, Ben Kahn. She pulled out a tray of sweets — “vegan, gluten free,” she noted — and organized a rendition of “Happy Birthday” as Mr. Kahn, 32, who has known Ms. Nyong’o since their college days, grinned. He was being feted at a glittery, exclusive soiree here, surrounded by Hollywood luminaries. That Ms. Nyong’o, the breakout star of “12 Years a Slave,” had hours before won an award from the Screen Actors Guild, for her supporting performance as Patsey, the tormented young slave in that Steve McQueen drama, did not seem to muddle the celebration. She got teary in her televised acceptance speech, then sang and lit birthday candles at the after-party, still dressed in her turquoise Gucci gown. A steady parade of actors turned fans, like Michelle Dockery of “Downton Abbey” (they share a stylist) and Jesse Tyler Ferguson of “Modern Family” (he voted for her, he said) lined up to pose for photos and deliver kudos. In between, she and her “12 Years” cast mate Benedict Cumberbatch danced to a Lionel Richie song.

It was another laurel for Ms. Nyong’o, who grew up in Kenya, lives in Brooklyn and has spent the last few months popping up on red carpets and at awards ceremonies, being lauded for her bold-hued style. As one of the five best supporting actress Oscar nominees, Ms. Nyong’o is the most traditional ingénue — 84-year-old June Squibb, a star of “Nebraska,” is the unexpected one — and a front-runner for the prize. Her chief competition may be Jennifer Lawrence, a star of “American Hustle” and the reigning breakout salty-sweetheart. “12 Years a Slave” is Ms. Nyong’o’s first feature film.

In November, she was still coaching people on how to pronounce her name and cooing over meeting Oprah Winfrey. “She gave me a big hug, which was just sooo yummy,” she said at the Gotham Awards in December. “Basically, my life is complete now.” By January, when she won best supporting actress at the Critics’ Choice Awards, the screaming photographers were on a first-name basis, and she and Ms. Winfrey were embracing like old chums.

She has accumulated more than 128,000 followers on Instagram, where she posts photos of her starry exploits.

If Ms. Nyong’o, 30, has made a smooth transition to Hollywood acclaim, it’s perhaps because she’s had training. A 2012 graduate of the Yale School of Drama, she stepped into the role of Patsey, the slave obsessed over by Michael Fassbender’s sadistic plantation owner, with a deep understanding of the character.

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