Catherine Denueve, who got impatient waiting for the Yves Saint Laurent show to start Monday night, said she’s busy promoting her latest film “La Fille du RER” (“Girl on the Train”), which comes out next week. New mom Ludivine Sagnier said she’ll ease into work again on the stage this summer, playing the wife of a very cruel king. “She’s a modern Desdemona, with balls,” Sagnier said. A few seats away, Joana Preiss took time out from spending hours in makeup for her latest role. “I can really let rip, it’s a futuristic horror movie,” she said.