Once again I will recommend my most favourite book ever - "Impro" by Keith Johnstone.
Only because he explains how I feel about the inner workings of characters far better than I can.
I am a character animator by profession, as well as being of the amateur writing sort.
Characters have
always come first over plot for me, as I find that if I don't enjoy the character (i.e. - find any emotional investment in that character) then I don't care about what he/she/it is doing - no matter how grand the plot.
Plots are bones, characters are the soul.
I am always willing to let a character grow on me - therefore, they don't have to be fully explained right from the get-go.
I think only a minimum of explanation is required to start with. Their name, and their current state of mind (preferably as reflected in their actions rather than outright described).
Action with inner monologue is fine, although I'm not fond of overexplanation for why a character is doing something - it's like being led by the hand, as if I were incapable of being able to realise the relevance of that action, later on in the story.
Evolution is good - this comes from seeing how they react off other characters/situations they are introduced to.
When writing, I find it best to visualise the scene and ask myself what the most obvious thing for them to do, is (as befits what I already know of their character), or if I feel "yes, this (insert new train of thought/action) is still in keeping" with their nature, I will go with that.
Obviousness is
very important.
Too many people try to be clever - when the obvious answer is usually the best, and feels right.
"Being clever" often comes from a fear of being unoriginal.
Being obvious and logical will still just as likely lead you to more interesting heights than you can probably imagine, and yet will still feel right.
The "different" sometimes doesn't sit so well - hence the awkward feeling - this is where the author is forcing their will over the fundamental nature of the character.
Writers should collaborate with their characters.
It makes sense to "listen" to your character (in the context of the story, that is!.

)
On building -
If you start your character from simple solid basics, i.e. their name, and general attitude to life, then you have a good base to build on.
If you have too many ideas about what your character might be, before you get to know the character, this can turn out to be messy.
Sometimes going through the messy route, a notion may come up - whereby you preserve that notion, discard the rest, and then build your character from there, me, I prefer starting from scratch to see what evolves. Either way works, but the simple method is quicker.
Your character's back history will evolve as you learn more about the character.
The possibilities are endless
Thar be my two ducats 'orth!