Originally a "The" preceded the title, until television producer Stuart Reynolds threatened to sue Rod Serling for the use of the name because at the time he was selling an educational film of the same name to public schools. Reruns following the initial broadcast featured the title screen "The Private World of Darkness".
Because CBS consulted different prints over the years for syndication packages, the closing credits vary from one side to side depending on what TV station is used for what version, and made a complicated job for the filmmakers. In The Twilight Zone (1959)'s original DVD release, the syndicated version was marketed as an "alternate version".
The trivia item below may give away important plot points.
The director had planned to have Maxine Stuart, who spoke all of the lines of the main character Janet Tyler when her head is entirely covered by bandages, dub in the single line spoken by Tyler when she is revealed portrayed by the actress Donna Douglas. However, Douglas had been listening to Tyler's voice as she recorded her part and was able to imitate her so successfully that she was allowed to speak the line on camera herself.