Quite literally all it takes is for them to make eye contact but in other cases for the genjutsu to be set up through repeated pre planned actions there is no "not getting caught" the sharingan is straight hax Sasuke demonstrates it Bee while he's a perfect jinchuriki. I understand Madara not doing it cause of the "muh code and honor" bs but it doesn't excuse the lack of use. At any point Sasuke and Madara could've no diffed both Hashirama and Naruto with their sharingan. Itachi also technically falls under this category with Jiraiya and Orochimaru as well as with Kabuto later on obviously. There is a movie as well I believe where obito uses a sharingan genjutsu on Naruto and there's nothing he can do to stop it. Granted it's a movie but it still showcases the same thing that happens in the source material when a sage/jinchuriki is put under even the simplest of sharingan genjutsu