Well the point of the huge names is to commercially offset casting an unknown for your lead. The Kents, Jor-El and Lara and Perry White are roles tailor made for big names because you only need them for a few days of filming so they don’t break your budget, and they add credibility and appeal to other demos for your movie. It’s wise to keep to that, imo. Get the cool character actors for the roles that will be around for a while and actually have a variety of meaty scenes and material to work with, like as villains and various major figures in Metropolis.