Auditions, screen tests, cutthroat competition: Catherine Steadman is familiar with the challenges of the acting biz. In her new book “The Disappearing Act,” the 34-year-old actress best known for playing Mabel Lane Fox, one of Lady Mary’s rivals on “Downton Abbey,” puts her experience to good use. Her perspective as an actress and a writer bring realism to this novel, as she dips into Hollywood’s murky waters where sabotage, sexual assault and betrayal fuel the dream machine. “The Disappearing Act” — Steadman’s third novel, following “Something in the Water” (2018) and “Mr. Nobody” (2020) — is a psychological thriller that evokes the seamy side of what it takes to become a Hollywood star.