Director Sidney Lumet once gave wonderful advice to aspiring directors: make sure everyone is making the same movie. In short, ensure that no department is isolated from the rest of production, working in private, doing something that may not be linked to everything else. Gunn noted that his upcoming film “Superman: Legacy” was focused on a unified aesthetic. He wanted the hairdressers, prosthetic makers, and production designers to all be on the same page. This was something he also did with his films “Suicide Squad” and “Guardians of the Galaxy, Vol. 3”, both huge and expensive. Gunn said:
“[O]One thing I noticed early on, when I started directing bigger films, is that I would watch other big films – I don't want to name them, but – occasionally I would see another big film where it was like, well, you can say that the costume designer I wasn't on the same page as the set designer, I wasn't on the same page as the hairdresser, and those, they're all individually great jobs, but together they don't fit together. […] [T]here's an aesthetic that you want to, you know, stand against everything.”
And when the goal is aesthetic unity, it might be good to leave the cast a little loose. If Superman has everything to do with the look of Metropolis, the cut of the costume, the hairstyle, then it's less important which actor goes in there. In fact, ask yourself if you would watch a Superman movie where he didn't wear his costume. Superman, Gunn seems to recognize, is more a fantasy than a man.
In fact, Gunn has explicitly said that actors are not a priority for him.