Warner Bros. has yet to officially comment on McNairy, Mulvey, and Malone's roles; though multiple sources have suggested Mulvey is the head of Luthor's security (meaning: not a fit for the "opposite" character that Eisenberg declined). Recent rumors have claimed that Jena Malone is portraying former Batgirl-turned-Oracle Barbara Gordon but the actress has also been rumored to play female Robin, Carrie Kelley. Still, as we've learned more about Batman V Superman, it's becoming harder to see where a different Robin would fit into Batman's arc - without convoluting backstory of the male Robin suit seen in the film's trailer. That's all to say, while it's possible that Eisenberg could have been up for an earlier Robin role, before the filmmakers knew exactly which version of the hero they would use (or where he/she would fit), the more likely scenario is that Eisenberg was up for the part now occupied by Scoot McNairy.
Like Malone and Mulvey, we've heard different things about McNairy's role. Initially, on-set images featuring the actor in green (screen) socks, sparked speculation that he'd be playing The Flash - until Ezra Miller was cast as Barry Allen. At that point, rumors began to surface that McNairy would portray a formerly injured scientist (or possibly Robin) that was augmented with cybernetic legs after injury during the climactic fight in Man of Steel. If the character was a scientist, or related to DC's scientific community in some way, many believed that McNairy's character would be a pre-cursor to the transformation of Victor Stone into Cyborg. While that is still a possibility, and could have been the role pitched to Eisenberg, speculation that McNairy will play iconic Daily Planet photographer Jimmy Olsen have begun to take hold as of late. Skeptics will point to the "Jenny" that was already established in Man of Steel's Daily Planet as an indication that Jimmy won't appear in DC's cinematic universe - though that character has since been confirmed to be Jenny Jurwich (not Olsen), meaning that there's still room for Jimmy.
As a Snyder "secret," we cannot be sure that Eisenberg was originally approached to play Jimmy Olsen (who may now be played by Scoot McNairy). In fact, we have no way of knowing that the the character that Eisenberg declined is even in Batman V Superman anymore; yet, Jimmy Olsen does seem like the most logical fit. After all, if we heard that Dawn of Justice was looking to cast a Jimmy Olsen back in 2013, it's easy to imagine that Eisenberg would have appeared on plenty of fan-casting lists - meaning it's equally easy to imagine that producers might also have been looking at Eisenberg for the role.
Of course, given the amount of fan-cast lists that wanted Bryan Cranston for Lex Luthor, it may be true that Snyder and his team weren't always looking for the most obvious choices. Yet, the fact that Eisenberg passed on his original offer and that Snyder re-approached him with Lex Luthor (even de-aging the villain to fit the young actor) - believing Eisenberg could do something special with the character - should help calm some of the anxiety that moviegoers have about the LexCorp head in this DC movie universe. Out of context, it's fair for viewers to be a little unsure of what to expect but with Snyder promising that the Dawn of Justice trailers are only a small glimpse at the film, with many secrets yet to be revealed, it'll be interesting to see how Eisenberg's take is received when audiences get the full picture.
Batman v Superman: Dawn of Justice opens on March 25, 2016, followed by Suicide Squad on August 5, 2016; Wonder Woman on June 23, 2017; Justice League Part One on November 17, 2017; The Flash on March 23, 2018; Aquaman on July 27, 2018; Shazam on April 5, 2019; Justice League Part Two on June 14, 2019; Cyborg on April 3, 2020; and Green Lantern on June 19, 2020. The Superman and Batman solo films have release dates TBD.
Source: LA Times