I think that only applies if you can make an argument that the two characters are distinct. Your Mickey/Steamboat Willie example is good because they are distinct (slightly different looks, and different names). Another good example is Sherlock Holmes. There was a big lawsuit where the current rights holders tried to argue that the later works are still under copyright just because Sherlock has emotions, and he didn’t in the earlier stories. I don’t remember how the suit turned out though.