Copyright law in the US has a preemption clause specifically intended to prohibit people from constructing copyright-shaped legal claims out of things that aren

Copyright law in the US has a preemption clause specifically intended to prohibit people from constructing copyright-shaped legal claims out of things that aren't actually copyrightable[0]. So you probably couldn't sue someone for trademark infringement purely because they stuck a hypothetically public domain Mickey Mouse in an otherwise unrelated book, or remixed the crap out of Steamboat Willie. As long as you avoided implying that your work was an official Disney product, they probably wouldn't have a claim. What you would have to worry about is the split between public-domain Mickey and copyrighted Mickey. The minimum standard for copyright infringement is access plus substantial similarity, so you'd have to make sure to either... 1. Only ever watch public-domain Mickey Mouse shorts, and hope nobody can argue access to the still-copyrighted works. This sort of sequestration would be rather difficult to pull off; though fortunately the whole "inverse ratio" nonsense that got Katy Perry in trouble has been overturned. 2. Intentionally watch all of the copyrighted shorts and use that knowledge as a guide of "what not to do". What actually constitutes substantial similarity is rather difficult; it's typically something juries decide on a case-by-case basis. You'd be best protected if you were doing something entirely different to what Disney was doing with his[1] own characters. The idea behind substantial similarity is that if I can squint at your copy a little and see the copyrighted original, then you're not legally distant enough to be a separate work. So you want to make sure to keep novelty in mind when reusing old Disney works. Things that would be "obvious to try" likely have already been tried and are thus still under copyright[2]. [0] The intended target of copyright preemption was actually state law; pre-1973 states had their own legal regimes for sound recordings which didn't get struck down until just today. Interestingly enough these common-law sound recording regimes also sidestepped the whole "for limited times" things and were actually perpetual in some states. That's why so much music is hitting the public domain today. [1] Historians of early American animation would probably argue that Disney did not create Mickey Mouse, but that Ub Iwerks did. This is correct; but you must also remember Walt Disney's villain origin story of having Ub's prior work on Oswald taken from him. Disney insisted on copyright assignment or work-for-hire status for everything afterwards, and that's what actually matters legally. [2] This is the same reason why it's legally perilous to use public-domain Sherlock Holmes as a character; the stories where Arthur Conan Doyle decided to let Sherlock emote are still under copyright, so the character's emotions are copyrighted, too. This is an absolutely silly legal argument that the Doyle estate has actively pursued in court.