The real "Sybil" was identified in 1998 as Shirley 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.
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Sally Field was the last person to audition before the director's lunch break, and Field's agent had to plead with them to let her audition. Ultimately, Joanne Woodward decided she wanted to see Field audition.
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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.
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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.
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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.
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