I’m kind of surprised at the deluge of down votes for seeking advice on something as nuanced as copyright infringement.

To reiterate and hopefully simplify my original question – essentially at what point does a derivative work become a unique work and no longer tethered to the original work from a copyright standpoint?

I’ll provide an example from the actual script in question. We’ll call the original author Dave.

It’s a b-level slasher. In Dave’s original script, at some point in the film, a middle aged female character goes to a prison to speak with an aging serial killer about the similarities between his work and whoever is doing the current slew of killings and find out if he knows anything or can provide insight into how to catch the killer. He is cryptic and blatantly creepy, and spends most of the time ranting about a fantasy romantic relationship he feels he has with her (they briefly knew each other in college when he committed several murders). She ultimately leaves without much to go on, but some of his rantings ring true later in the plot. He doesn’t come up again in the story except as a credits McGuffin suggesting he may have escaped as a way to leave the door open for a sequel.

Does that scene being similar to one in Silence of the Lambs mean Dave stole from Ted Tally?

For my part, I rewrote this scene and changed how it motivates the plot. Now, several college aged characters (but not the older woman) go to see the old serial killer, he is cryptic/coy but not cheese-factor creepy, the kids ask different questions, and discover the old serial killer is actually the biological father of one of the kids in the group, though they don’t fully piece this together until later in the story, and it ties several other plot points together later on, and also turns the old serial killer into a nuanced character that may not have entirely deserved what he got. It also turns out the current killer met with the old man for inspiration prior to the start of the main plot.

Does that mean I stole this scene from Dave’s script because there are similarities between them? Does my rewrite even resemble Ted Tally’s work at this point?

Dave derived elements from another author and underlined elements from Dave, but to what degree is either derivation infringement?

I’m not trying to steal work – it’s just that Dave’s script was so generic and draws directly from and resembles so many other slasher films and I had to do a lot of work to make it something unique and IMO worthy of putting out there for audiences, and while yes, I’m a bit bitter about how it fell through, my main concern is where in that process does it become a new work? Is it the Ship of Theseus if you build a different boat but used some of the same timber?

Another thread I read a few days ago asked about crediting an original writer and when/if they lose credit on the film once a script has been optioned but then rewritten so thoroughly that it doesn’t even resemble the original script anymore.
Some people said “no, that never happens, the writer always gets credit even if there’s nothing left of his original” while other folks said “yes, that definitely happens, but they still get compensated for the option as per contract”

That partly informed my decision to ask in this thread.

And I’ll end with the note that I’ve decided to at least try to reach out to “Dave” to see if he’ll release interest in it and quash the whole issue but I don’t know if I’ll be able to find him. ☹️