Milly Alcock found out she’d booked the coveted role of Supergirl in one of this year’s biggest hits when “Superman” director (and co-CEO of DC Studios) texted her a link to an article announcing her casting. She admitted she had a moment of doubt. “My first thought was: What have I done?” Alcock told a packed audience at the Newport Beach Film Festival. “It’s just scary.” But she enjoyed her cameo in “Superman” and went on to shoot her own film, “Supergirl,” set to be released next year. “I’ve