To be a celebrity is to negotiate a gulf between private self and public image. “It’s a bit of an isolating problem to have,” Ms. Williams said, and so she was gratified to find that this was among the themes of Milan Kundera’s novel “Immortality,” which she read on the plane to Cannes, though she said she wasn’t sure she agreed with the book’s conclusion, “that the self people perceive is just as real because it exists.”