When Zack Snyder says there is a “Robin line,” I think he means that Joker will have a line of dialogue that he says to Bruce Wayne’s Batman in Justice League that reminds him of the shared tragedy in their demented past. When might this happen? Well, there have been rumors that in a Knightmare sequence in Snyder’s Justice League, Batman (Affleck) will be fighting alongside Joker (Leto) and Deathstroke (Joe Manganiello) in the broken version of a future where Darkseid has destroyed our planet. If Snyder is actually going to show us that bizarre team up, I can see Leto’s Joker needling his “teammate” by reminding him of his greatest loss, his deepest failure. Such a Joker thing to do.