According to the Daily News, when Streisand returned for the second act following the show's intermission, she was spotted by a fan; when word spread that she was in the audience, the crowd stood for her.
Barbra Streisand celebrted her 73rd bday in NYC. Philly friend saw her in audience of An American In Paris, where they gave her a standing OStreisand had to wave at the crowd to sit down, so the second act could begin. The Daily News also reported that she went backstage after the show to say hello to Tony-nominated actor Robert Fairchild and the cast.
— HughE Dillon (@iPhillyChitChat) April 28, 2015
Streisand received a Special Tony Award in 1970.
An American in Paris was the most Tony-nominated production (alongside Fun Home) this season, with 12 nods including Best Musical. Click here for a complete list of this year's Tony nominees.