“Regretting You” (in theaters), the director Josh Boone’s adaptation of the Colleen Hoover novel, put Williams face to face with the teenage version of herself. She plays Morgan, a woman in her 30s who became pregnant at 17. A family tragedy compels her to reassess her life and her relationships with her daughter, Clara (Mckenna Grace), now 16, and an old friend, Jonah (Dave Franco). In flashbacks — with careful lighting and slight de-aging — Williams also plays Morgan as a teen.