FYI, they are ordered by weight. Check http://www.tvmaze.com/shows/82/game-of-thrones/cha... for example, the more appearances a character has, the higher up it's listed.
This works for 95% of the shows out there. Only a handful of shows have such dramatic shifts to the main cast that an alternative should perhaps be considered. Another example that comes to mind is seeing the Brodies listed as part of the most important main cast members on http://www.tvmaze.com/shows/7/homeland



