Beginning at the summer of 1968 at the Bayreuth Festival, Nilsson was obsessively stalked by a much younger American actress and model, Nell Theobald, until Theobald's suicide nine years later in 1977. Nilsson recounted her experiences with Theobald at length in her memoir, La Nilsson, in which she referred to Theobald solely as "Miss N." The stalking incident was later featured in Opera News magazine and The New York Times.
Despite her worldwide recognition, Nilsson said she was nervous before every major performance. "Before a premiere, on the way to the opera, I'd hope for just a small, small accident, it didn't need to be much, but just so I would not have to sing", she said in a 1977 interview on Swedish TV. Nilsson often spoke of her limits. She said her voice was not a good fit with what she described as the softer textures and refined tones of Italian operas. Nonetheless, she sang roles in Italianate operas such as Donna Anna in Don Giovanni.
Nilsson's autobiography, Mina minnesbilder (My memoirs in pictures) was published in Stockholm in 1977. She retired in 1984 to her childhood home in the Skåne province of southern Sweden, where her father had been a sixth-generation farmer and she had worked to grow beets and potatoes until she was 23. In an interview in the mid-1990s, she appeared happy, serene and as unpretentious as ever. "I've always tried to remember what my mother used to tell me", she said. "Stay close to the earth. Then when you fall down, it won't hurt so much."