Rachel Bilson jumped at the chance to star in a three-part short film series directed by the legendary Chanel designer for Magnum ice cream, which is launching in the United States. “I was like, I’m working with Karl Lagerfeld. I could be doing something, like, really ridiculous and in, like, a big clown suit. It wouldn’t matter,” she said. Bilson, 29, best known for her role on “The OC,” has also gained recognition for her sense of style and writes a column about fashion for In Style magazine. For Thursday night’s premiere for the ads at the Tribeca Film Festival, she wore a Chanel dress and carried a Chanel clutch. “I was very young when I knew that I was into fashion,” Bilson said. She said she insisted on picking her own outfits at the age of 2.