I'm of the opinion that any reasonable character should be just like a person in real life... That sounds like an obvious statement, right up until I also say that most people on the surface are rather one dimensional, dull or flat. That's not saying that people are not complex, which they all are, yet in most cases we'll never know what the hidden parts of their past are, what quirks they keep suppressed, what motivates them and why their moment to moment personality is one way, and why they seemingly for no reason (though they have one), suddenly act out of character.
IMO, you're writing a story. Not a collection of biographies. An entire novel wouldn't cover the subtle events of a person's life that 'collectively' forms them. It's never one big thing (which makes a bogus character in my opinion). It's thousands of seemingly inconsequential events that gradually nudges a person to who they are.
That said, there is nothing wrong with adding this little tidbit or that, yet in the end, the person is SIMPLY who they are at the moment the story is happening... the end. It's when they DO act out of character that a reason could possibly be given... then again, perhaps not. Perhaps when that person encounters X-situation, for whatever their reasons are, they act Y-way. Why explain it? that's just how they are.
The pitfall I feel is when people try to make characters interesting, quirky, full of inner secrets and demons. Then they come off as more caricatures than characters because you can either write a biography or a story... not both in a reasonable sized novel, and even still the biography is just highpoints, not the day in day out grind that shapes a person and
gives them character.
There is a FANTASTIC speech from a fantastic movie. Here is a short clip of that speech:
(By the way, if you want to learn how to write great dialogue, watch 'The Big Kahuna)
Okay, so with all of that out there, I recently dealt with a case in point during alpha reading of The Abolitionist.
My protagonist, the person who we never leave their side throughout, demonstrated numerous quirks, severe PTSD, social dysfunction then countered with social responsibility. Humor, dark brooding, rage, and numerous uncontrollable emotions and responses. Vague reasons were given... yet it was not until the next to the last chapter that I gave detailed reasons from her past granting it all justification.
Know this as well, the character (as in how she is), her past and those experiences are not fictional. They are all cut and dry and explain the whys of the character.
In that chapter, I laid it all out extremely well. In a way that each experience, which formed X-aspect of her personality, gave justification for her actions throughout, her ultimate conclusions building to the big climax, and tied in perfectly mirroring the events of the moment. In the end it made this intricate complex life now suddenly seem that it had reason on par with formative destiny.
The alpha readers hated it. In fact, it was the only part they disliked. I had whole chapters that read (to me, and intentionally) like dull lectures... those they liked. Yet when presented with real justification and facts outlining the eccentric 'whys' of the character... more so how it all applied to the moment and the problem at hand... my readers felt it was all unnecessary and detracted from the story.
My readers accepted her for who she was, at face value, and didn't need or want justification. The chapter was tight, interesting and exciting. But, it suddenly, though briefly, became a biography that they felt no need to hear. So, I cut it all out.
That's how we all live and deal with people everyday. We don't need detailed explanations as to why they do this or that or think this odd way. They simply do. We can either accept that, or, we keep our distance if we can't. Today, this moment, it doesn't matter 'why' someone is a particular way. All that matters is that they 'are' a particular way... because that is how
we will judge them, and can never know the collective reasons as to their 'why.'
In the end, that vague understanding makes them like real people. An intricate/intimate understanding, makes them a character.
Just my inexperienced opinion.
K2