As others have said, I don't quite approach it this way!
I do usually start with a rough concept for a person, like the idea of a character who's nice and wants to help but is very shy, and I guess you could boil that down to a couple of conflicting traits if that's how you think of it -- a) helpful, b) shy -- but when I develop them further, it isn't by adding more traits; it's by sort of expanding on what they want and what they're afraid of and how their ideals come into conflict and how they express them. Someone who's "shy" might find social situations unpleasant, or they might be self-conscious because they have low self-worth, or they might be nervous of doing the wrong thing. If your character is "shy," what fuels your character's shyness?
I think knowing that inner reason is more important than like.... having Some Event In Their Past That Caused It or whatever.
If you want to think of it more tangibly, I often look at this as like.... desires and fears. They're sort of like "traits" but, sometimes traits end up not going very deep. Knowing that your character is stuck-up about her intelligence doesn't really tell you much about her. But if you look at it in terms of desires and fears, knowing that your character is driven by a desire for knowledge, and just wants to learn as much as possible, and has trouble seeing eye-to-eye with people who won't put in the effort she's put in -- or alternately, knowing that your character is driven by a fear of being seen as stupid, and overcompensates by flaunting her intelligence because she places all of her self-worth on her smarts and is horrified by the idea of NOT being as smart as she's expected to be -- THAT tells you something about this character and what drives her.
And just look at how many different kinds of characters you can get out of the same basic traits!! Just developing that out to be more than a Basic Trait, but an actual part of the character that runs deep, helps a lot.
I'm a big fan of the idea that most qualities of a person have both positive and negative sides to them.
Someone infuriatingly fussy and non-compromising is a pain in the neck to be around, but exactly the person you want in charge of a crucial project that can't afford any mistakes. Someone who hates conflict might be an amazing peacemaker and tremendously forgiving, but afraid really to rock the boat when something is actually wrong and needs to be addressed.
I do look at my character's flaws and shortcomings and unhealthy coping mechanisms and see things that they need to learn to approach differently, but a lot of times I see those flaws as rising out of a fundamental part of who they are, something that isn't 100% bad or 100% good.
Think about it.... if your characters are defined by flat traits that are either positive or negative, how do they grow? If you have a character who has the negative trait of "cowardly," how do they become "brave" without arbitrarily and suddenly changing the character? But if you have a character who has the fear/flaw of "afraid of messing up," then they can grow organically -- they will still be afraid of messing up, but they can learn that messing up isn't the end of the world, or they can find something they desire that makes it worth it to try, even though they might mess up. They you see them grow and change -- from seeming cowardly on the surface, to seeming brave -- but fundamentally, inside, their fears didn't magically vanish, they just have ways to challenge those fears now.
