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.
Patty Duke auditioned for the lead role, although was not cast due to a clash with the original director. Daniel Petrie later signed on to direct, but Sally Field already accepted the role.
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.
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 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.
Sally Fields 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 plea with them to let her audition. Ultimately, 'Joanne Woodward' decided she wanted to see Field audition.