Given the fact that Bruce Wayne actually recruits Barry Allen into the League, we can likely expect a strong mentor/mentee dynamic, with Batman and Flash teaching each other vital lessons throughout their mission.
By contrast, Barry Allen is new to the world of superheroes, and he brings a wide-eyed sense of innocence and optimism.
Zack Snyder’s Justice League will use the differences in their experience level to explore the mentalities that each of these heroes brings to their respective crime fighting style.
Ben Affleck’s Bruce Wayne is a veteran; he has been at this for decades, and he has seen the worst of humanity.