I can imagine in certain movies, where the 'scenario' is limited to 'physical activities' between a variety of men, women, and various combinations. I will say 'they tend to be recurring scenes in those' (no I haven't watched them all). Will they start suing each other because they (more or less) are doing the same deed, the very same deed found in various ancient books and amphora, for the past few thousand years?

Copyright really does protect against copying. Proving that you came up with the same idea independently provides substantial protection in a lawsuit. This is why big-name authors will hire people to screen their mail and remove all suggestions, so they can say with a straight face in court that even though Fan #5,243 sent them the exact idea they ended up using in their seventh book in some series, they never saw it and therefore did not copy it.

Obviously, if you create a page-for-page match for some 500-page novel this still won't save you, because no one's going to buy your claim to have stumbled on that independently. This isn't licence to do large-scale copying and then try to claim you had no idea that somebody else wrote the exact same book. But this is the exact sort of case where demonstrating that there was no way that the putative copies are actually copies means the lawsuit will fail.

Bear in mind I'm talking about the intended purpose of the law. There are borderline and questionable cases where you may feel, even with substantial justification, that the court ruled against this principle. But this is at least the clear intention of the system.

You have to prove copying to prove infringement. But because actual copying is often done in private, under US law, courts deem copying to have happened if you can prove both "access" and "substantial similarity". Substantial similarity on its own isn't enough.

https://en.wikipedia.org/wiki/Substantial_similarity#Substan... says:

> it refers to that level of similarity sufficient to prove that copying has occurred, once access has been demonstrated.