We now might have an understanding of how The Flash makes his entrance into the new DC film universe, and it looks like it had to do with Batman.
At least, Batman's nightmares. We've seen glimpses of these in the trailers, but Entertainment Weekly sheds some light on how these are handled.Both Clark and Bruce have various dream sequences throughout the film, but if they are just dreams, or visions, or hallucinations is unclear. The Flash eventually appears to Bruce during one of these nightmares, and Zack Snyder has big hopes for this scene.
Fans will have to debate whether it was really a dream, or maybe the kinetic hero breaking through time-space to deliver a warning. Synder calls this “a giant Easter egg” that he hopes will spawn plenty of online theories. “We had such a straightforward narrative, on one hand, I wanted this other layer of the movie to be complicated and bizarre,” he says.
We will have to wait and see, but I do like the experimentation aspect to this, and that everything isn't so straightforward. My money is that he's real, and possibly someone the Dark Knight has recently seen or encountered, hence, the reason he is in his dreams.