I think just jumping into a Justice League movie is a huge mistake. Everyone knows Batman and Superman, but the others are all basically unknown. Some might include Wonder Woman, but she's really not known outside those who actually read comics. Sure, everyone and their dog knows Wonder Woman, but you go ahead and ask any of those people to tell you anything about Wonder Woman other than the fact she was played by Linda Carter, they'll stare at you blankly. To the public Wonder Woman is essentially: Linda Carter, lasso, invisible airplane. An it's easy to see why. Superman and Batman have fairly straightforward concept, origins, powers, but Wonder Woman has so many conflicting ideas involving mythology, patriotism, feminism, and BDSM that no one really knows who she is.
The others aren't much better. Flash is fairly simple, but Green Lantern is an inherently silly concept that will probably need some explaining, and don't tell me they won't need to explain it since he was in a film just a bit ago. That movie is like the sixth biggest flop ever so they can't really expect people to know about Green Lantern from it. Martian Manhunter, Aquaman, Hawkman, Cyborg and others are inherently strange and will probably need a fair amount of setup as well. And Green Arrow is easy as hell but would have the stupid public complaining they were ripping off Hawkeye.