As everybody knows, Batman v Superman features a nightmare sequence in which Batman sees a possible future, one that will bring Darkseid to Earth. However, this isn't the only dream sequence to appear in the movie. According to Entertainment Weekly, Superman will also have one, while the Dark Knight will have one more.
Who is it that brings these visions? That hasn’t been revealed just yet. However, EW reports that one of Bruce Wayne's dreams will also feature the Flash, who comes to warn him about an impending threat. Apparently, this is all that Flash will do in Batman v Superman - so it isn't as much as some fans believed.
Now we can, of course, speculate about this dream appearance. According to some, Flash will use the Speed Force to deliver his messages. Which means that Batman v Superman will see the appearance of an already veteran speedster, one who actually knows how to make the most of his powers.
According to Zack Snyder, this appearance of the superhero is intended to be a HUGE Easter Egg that should bring in a lot of speculation. In a sense, it is meant to get the fans even more interested in what is to come for the DCEU. This new piece of information (which can be taken as official news) contradicts previous rumors that said the Flash will be just a blur in one of the Batman v Superman scenes.
Warner Bros has also released a new TV clip for Batman v Superman, which can be viewed below. While it doesn't have a lot of new footage (How much can it have, in little less than 30 seconds?), it does show that Batman has found a way to fight Superman.
https://www.youtube.com/watch?v=SJt-kMXYItA
Just 21 days until Batman v Superman is released!
If you want some more, EW also shows the Batman who always comes prepared to a fight, somewhat like a chess player, able to anticipate every move made by the adversary. And this is an interesting change from what we have seen so far.