According to the DVD commentary by Aubrey Solomon, "When the real Eve signed the contract for the movie, the studio had her sign three separate contracts under each of her personalities, so that they would be covered from any possible legal action. In fact, when she signed the contract, they actually had her go into the personalities, and the signatures were all different on the contracts."
This story of Multiple Personalty Disorder was followed by Sybil (1976), another story of multiple personalities. Joanne Woodward, who played Eve here, was cast as Dr. Wilbur, the psychiatrist who treats Sybil.
The film is based on the true case of Christine Costner Sizemore, who later wrote two books about her multiple personalities: "I'm Eve" (1977) and "A Mind of My Own" (1989).
In her 1977 book, "I'm Eve", Christine Costner Sizemore, the woman on whom the character of "Eve" is based, said that she really had 26 multiple personalities at the time, not just three; and that the popularity of the book and movie based on her life only traumatized her further as Dr. Corbett Thigpen forced her to sign over all rights to her story. Contrary to this statement, Dr. Thigpen personally made sure that Mrs. Sizemore was given a fair share of any and all profit from the movie. Furthermore, Dr. Thigpen and Mrs. Sizemore were on good terms with each other before Dr. Thigpen passed away in 1999.
Orson Welles was courted for the part of Dr. Luther, played in the movie by Lee J. Cobb, but opted instead to direct Im Zeichen des Bösen (1958). Welles even read the script and told director Nunnally Johnson that whoever played the lead would win an Oscar. (Joanne Woodward did, in fact, win the Oscar.)