In character development, as in a lot of other aspects of writing, having a good reader is an essential part of the writing process. A response helps clarify if you have communicated what you intended to, or if you have written down quite another world than the one inside your head.
Classic writing class instruction includes the axioms “Show, don’t tell,” and “specific is terrific.” These have good ideals at their heart, especially concerning character development. Only Republican presidential debate transcripts are more agonizing to read than paragraphs of development through description. “Mary Sue was a brave girl, intelligent, but understated. She enjoyed eccentric clothing and pop music of the indie persuasion, and her kindness was obvious. But she had a dark side, too. Mary Sue was a passionate mix of the good and evil that lies in all of us.”
However, if an author eschews explicit narration altogether, s/he may find that the audience takes away some surprising notions about a character. This is where a good reader comes into the mix. My brother is probably my best reader (as my husband has to be more supportive than critical), and after he read one of my novels, he asked me, “Was the judge supposed to sympathize with the government or the protestors?” I launched into a tirade of explanation–how could this have even been in question? Alas, if I wanted the audience to understand the judge’s motivations, I would have had to tell them what they were.
When I’m writing, I always know if my character is inscrutable or creepy. Sometimes I don’t let my audience know, though. I know who I meant to introduce; only my audience can tell me who they met.