Commentary
– But whether a character (as opposed to a work) is in the public domain is based on the idea that its first appearance is in the public domain and all subsequent appearances are considered works derivative of the first (and whether later changes to a character can be used is based on whether the first appearance of those changes has entered the public domain, like if a character is later described as a detective or a king or the brother of another character).
Elements original to the public‐domain Captain Video comic book can certainly be used, like the original artwork and new characters, but not the Captain Video character itself if the first episode of CV&HVR is not in the public domain. (Similarly, Popeye is not a public‐domain character even though some animated Popeye cartoons from the 1950s have lapsed into the public domain.)
I think it’s been found in court or it’s been stated by the copyright office that broadcast alone does not constitute publication because physical copies have not been distributed (as opposed to cinematic release, wherein actual copies of a film are distributed to theaters). This is why the original Star Trek series is not in the public domain even though they were aired without onscreen copyright notices, which would invalidate the copyright of a film. Unfortunately, I have been unable for a long time to find where I read this.
This is all complicated by the fact that (I believe) the first episode of CV&HVR is not only lost but may never have even been recorded in the first place on any medium at all, being only a live broadcast, possibly without even a script. (22:14)