By learning to care about one another (and apparently bust each other’s chops, which was the case with one or two of the other actors) in boot camp, they managed to achieve a real sense of chemistry.
Becuase of that, now they even keep in constant contact in a group chat like a group of high school friends.
So in their efforts to quite literally build themselves into firefighters, these actors were able to forge a bond reminiscent of the real men who battle wildfire blazes.
That is a critical element to establish in a film like this because much of Only the Brave’s most dramatic moments stem from the palpable sense that these guys care about each other.