The actor and filmmaker Griffin Dunne learned that his aunt, the writer Joan Didion, had become the new face of the fashion brand Céline the same way the rest of the world did: He saw the picture on the internet. He was traveling at the time, and all of a sudden, he said, “my phone exploded. Everyone was calling about that photo.”

The 2015 photograph was certainly striking: Ms. Didion was perched on her sofa in a black turtleneck, her customary blunt silver bob, and sunglasses so large they obscured half her face. Mr. Dunne had no warning about the advertisement, even though he had been at work directing a documentary about Ms. Didion’s life produced by his cousin (and Ms. Didion’s grandniece) Annabelle Dunne. Although they had been spending hours interviewing Ms. Didion, now 82, she kept the shoot a secret.

“But that’s Joan,” Ms. Dunne, 34, said, sitting on a recent morning in her cozy West Village townhouse with a bespectacled Mr. Dunne, 62, drinking his second cup of coffee. “She’s still really savvy. She recognizes good style, and how to create a moment.”

Ms. Didion was also savvy about whom she chose as her documentarians. She had been approached several times by filmmakers, but she turned them all down, Mr. Dunne said. It was only when she felt that the movie could stay within the family that she allowed unprecedented access to her archive and granted her nephew a series of frank interviews in a process that proved personally wrenching for him. As a member of a family that has been particularly touched by loss and grief, Mr. Dunne had to experience past tragedies all over again in order to make the film.