I guess you have to start with tv movies eventually!
Namaste, and good luck.
If you've seen the Sybil (1976) extras dvd, you'll understand that the original movie isn't based on the book, but on Stewart Stern's own research on the case, along with technical advice and input from Dr Wilbur herself. As he said (paraphrase) he knew he couldn't write a screenplay based on the book, so he met with Dr Wilbur, obtained tapes and session notes and wrote the story "right down the middle." Sybil's "boyfriend" Richard is a dramatic person used to show us how Sybil might interract with others and how the characters might come out to the non-professional. He's a projection of Danny, who was a friend of Sybil and might have become her husband but for circumstances. He moved away to Texas when she was 12.
There's been a lot of research and controversy surrounding the issue of Multiple Personality Disorder (now officially Dissociative Identity Disorder, DID) in the 30 years since Sybil was published and the original movie shown. I'm not going to go into that topic, but there are numerous posts on both of the Sybil boards if you're interested. The topics of DID and Sybil have remained pretty hot ones, especially after it was learned that her true identify was Shirley Ardell Mason, a former college art teacher and artist who lived the last years of her life in Lexington, KY. (She died in 1998, Dr Wilbur passed away in 1992, and Flora Schreiber, the writer, in 1995 IIRC.)
There's still a mine of information about Sybil in the book that was untouched; most of her childhood, her early dissociations, early treatment, college years, the real Danny, her grandmother's death, the real struggle to reintegrate (over 3 years, not an afternoon in the park,)Hattie and Wilbur's life, and much much more. There's plenty of information to make a new movie without being a "remake" of the original.
I've never seen Tammy Blanchard perform, but I do like and respect Jessica Lange. This was exactly the case in 1976; I had never seen Sally Field (I was too busy with other things to be a "Flying Nun" or "Gidget" fan) but was familiar with Joann Woodward. I was awed by both of their performances and many scenes stayed with me until I finally got the dvd in August and was once again captured by the story and their performances. I don't know what to expect from Blanchard/Lange, but the story of Sybil still enthralls me and I for one am looking forward to this movie coming out.