I see scripts where the characters have a lot of quirks, tics, & other unusual behaviors, but lack traits. The writer seems to think the more quirks, the better to hide there's no three dimensional character.
Quirks are like lace doilies, i.e., they should be used sparingly. I wince at the thought of a doily laden table. Think of how I'd stomp all over a doily laden script.
Traits, on the other hand, are the bricks that build the story. They're not as exciting, but hold steady, because there is a purpose, a long gestating reason that characters have them.
Quirks are only good in the moment & can be used for spice & surprise.
Traits can play out over a series of scenes & usually tell you something more...gives you a foothold for subtext.
Quirk - Woman eats only white foods at the big banquet.
Trait - Woman cheats in her food journal about how many calories she eats (a la Bridget Jones' Diary).
Quirk - Man will not step on a crack.
Trait - Man avoids all authority, including traffic signs, and jaywalks deliberately.
WHAT I'VE LEARNED: Don't doily your scripts. Please. Restrain yourself.