Steve Buscemi almost turned down his role for the exact reason the Coen brothers wanted him in it
On his first read of the Coen brothers script, Steve Buscemi was dismayed at the treatment his character Donny received. He just could not wrap his head around the abuse Donny’s friend Walter constantly barraged him with, to the point where he became uncomfortable with the idea of performing in the film. Out of respect for the Coen’s work, he went ahead and read the script through to the end. In the final act of the film, the way Walter attempted to protect and then later eulogized the fallen Donny moved him, and he was sold. Seeing the whole character arc play out, he realized a brotherly connection between the two characters that sealed the deal for him.
Incidentally, all that abuse that initially turned off Buscemi was absolutely intentional by the directors as an in-joke. Buscemi had previously played Carl Showalter in Fargo, and in that film his character was constantly talking. Just would not stop. The idea of having a character constantly screaming at Buscemi to shut up in their next film was deliciously funny to the Coens, who included it in the script.