I am curious to know what everyone thinks about supporting characters.  Specifically, how deep should they be?  Now that I've actually done some writing on my novel, my characters are appearing more clearly.  I am doing a lot of revising to their "character sheets" to help me better understand them and flesh them out.  I am having trouble with a supporting character's motivations and conflict.  It occurs to me to wonder whether I really care.  Her role is basically that of best friend to the primary character.  She will be helping the protagonist and doing some mediating between primary and secondary character.  She fills an important role, but the story is not really about her in the slightest.  It's about my primary and secondary characters, mostly the primary.

Thoughts?