We already know that Ben Affleck's Batman will have a key role in Justice League, as he will be working to redeem himself by assembling the superheroes. As he revealed to Wonder Woman in the ending scene of Batman v Superman: Dawn of Justice, he will be seeking them out in order to unite them and form the Justice League.
During a recent visit to the London set of Justice League, a group of reporters witnessed the filming of a couple of scenes, one of which features Bruce Wayne and Barry Allen. This is the scene where Affleck's character recruits Ezra Miller's character.
A reporter asked Ben Affleck if The Flash would serve as something of a sidekick to the Caped Crusader in the November 17, 2017 release, as Robin has in the comics.
According to Comic Book Resources, the 43-year-old actor responded:
That’s interesting. There’s an element of that to it. There’s a quality to really what Ezra [Miller] does that is young and fun and full of life, and excited about what they’re doing that’s so in contrast to who Batman is.
The description of the scene in which Batman recruits the Scarlet Speedster does indicate the contrast between the two Justice League superheroes. While Bruce's concern about possible threats makes him assemble the heroes, Barry is simply glad to join if he gets to keep the batarang!
Affleck adds:
There’s a little bit of that natural yin and yang to playing scenes with him. There’s not the ward aspect to it, but there’s a little bit of the mentor – which you’d probably ask Ezra and he’d be like, “Fuck that! He’s not my mentor!” But I think there is, a little bit. And it’s fun to play, definitely.
When you have a Batman who has been in there fighting crime for more than 20 years, and a Flash in his early days with his superpower, that's the kind of relationship you would expect between the two.
