The real "Sybil" was recently identified as Shirley Ardell Mason, an art teacher who died in 1998 at the age of 75 in Lexington, Kentucky. Flora Rheta Schreiber, who wrote the book on which "Sybil" was based, gave her the name based on the women prophets of Greek mythology, the Sibyls, who spoke with multiple voices.
Sally Field almost did not get the part of Sybil. She was the last person to audition before the director's lunch break, and Fields' agent had to plead with them to let her audition. Ultimately, Joanne Woodward decided she wanted to see Field audition.
In searching for the lead role of Sybil, Joanne Woodward was originally wanted for the role. She had played a multiple personalities patient in The Three Faces of Eve (1957). However she declined and asked to play the role of Dr. Wilbur.
Natalie Wood was the casting director's first choice for Sybil, but the screenwriter turned Wood down. The role was then offered to Audrey Hepburn, who declined. The project remained dormant until Joanne Woodward agreed to co-star.
In the beginning minutes of the film, as Sybil calls to her art class of primary school children to pack up, she calls to "Peter" and "Eli". These are the names of Sally Field's actual children, who were seven and four years old at the time of filming.
Joanne Woodward, who plays the Doctor, starred in another famous film about multiple personalities, The Three Faces of Eve (1957) where she starred as the titular Eve. However, unlike Sybil, she had only a trio of personalities.
The screenwriter thought the role of Sibyl would best be played by an actress who wasn't extremely well known to the public, so they wouldn't bring along previous associations as to her personality. With that in mind, he believed an "interesting" choice would be Lily Tomlin, as she could also obviously create distinct characters for all the separate personalities. He did become completely supportive of the idea of Sally Field playing the role after he witnessed Field's audition, however.