A Trial of Characters

I have found it constantly fascinating over the years how some characters are easy to write and some are like pulling hens' teeth. Some characters can surprise you with their wit and wisdom and on more than one occasion they have surprised me with their vocabulary. Other characters sit resolutely in the background mumbling inaudibly. Who are these characters and more importantly, why are they here?

My usual journey leads me to discover that each of these characters is there for a reason - although not necessarily the reason that I first thought. It seems that no matter how much research I do, how much outlining and planning, the damn people in my stories are determined to tell the tale their way and not mine. I have learned that nine times out of ten they know best.

Have other authors had this particular dilemma? I'd like to know.