For me its all about trying to evoke a mood from the initial stage. Everything beyond that point is just an excuse to support the atmosphere/mood/narrative I'm trying to get across. Of course many times the elements (characters, props etc) are predefined, so that's an added challenge into finding a way to work the lighting in --and around-- what I'm given in order to achieve that mood.
Sometimes it can be value-driven... other times it can be colour driven (though I wish I had decent colour sense to be able to really push that aspect.. I think colour is more important than value sense, and much harder to forgive when 'done wrong', if you know what I mean). Once again all of it is to support the image as a whole, and I don't think any aspect is more important than the other. Ideally all can balance out with proper planning. Easier said than done. Hope that made sense!