But well-developed side characters would, inevitably, reflect or develop the brothers and their relationship and/or story, because developing them would require interaction with the Winchesters, which would in turn develop both the Winchesters (because you don't act with everyone the same way as you do your brother) and the world they inhabit (oh hey, other people exist, and they have their own motivations and goals and ways of living). Developed side characters also relieve the burden on a single character from giving the Winchesters answers too often.
I think the worst way to develop the brothers' relationship and story would be to limit their interactions to themselves.
Re: Extended TV Guide spoilers
But well-developed side characters would, inevitably, reflect or develop the brothers and their relationship and/or story, because developing them would require interaction with the Winchesters, which would in turn develop both the Winchesters (because you don't act with everyone the same way as you do your brother) and the world they inhabit (oh hey, other people exist, and they have their own motivations and goals and ways of living). Developed side characters also relieve the burden on a single character from giving the Winchesters answers too often.
I think the worst way to develop the brothers' relationship and story would be to limit their interactions to themselves.