According to a new interview with the New York Daily News, Batman V Superman: Dawn of Justice director Zack Snyder said that he never considered using Grant Gustin as The Flash, in part because the tone of The CW's The Flash simply didn't match what he wants to do with Justice League.
Instead, he went with Ezra Miller, whose take on Barry Allen will appear in what Snyder's wife (and the film's producer) Deborah Snyder has described as a cameo-like role in Batman V Superman.
"I just don't think [Gustin] was a good fit," Snyder said. “I’m very strict with this universe and I just don’t see a version where...that (tone is) not our world.”
There's one more intriguing reason Snyder gives for leaving Gustin out of the conversation: DC's multiverse.
“Even if Grant Gustin is my favorite guy in the world and he’s very good, we made a commitment to the multiverse (idea), so it’s just not a thing that’s possible.”
So, as Geoff Johns has hinted in the past, apparently all of these shows and movies do take place in the same multiverse...just not the same universe.
Fearing the actions of a god-like Super Hero left unchecked, Gotham City’s own formidable, forceful vigilante takes on Metropolis’s most revered, modern-day savior, while the world wrestles with what sort of hero it really needs. And with Batman and Superman at war with one another, a new threat quickly arises, putting mankind in greater danger than it’s ever known before.