If your character has a trait you consider one of their key features and it is never challenged in anyway, that trait will probably have about as much impact as their “tiny mole on their left buttcheek” trait.
For that trait NOT to fall totally flat, one or more of these things need to happen:
- The trait exacerbates a situation in a bad way. (An extremely honest and trusting character reveals an important secret to the Big Bad’s lackey)
- The character is given a choice to either act according to their traits or to subvert them. Often the subversion has the better outcome. They either maintain their trait and take a risk, or character development ensues. (A greedy character must escape. Leaving behind their riches goes against their very being, but it would allow them to escape easier. If they try to take some of their treasure with them, they do so at massive risk.)
- The character is put in an environment or must work with a person that opposes their trait. (An extremely tidy person must live in their slob cousin’s filthy apartment for a week. / The character has a debilitating fear of being alone and they are stranded on their own on an island.)
- The character is put in a situation that requires them to suppress their trait (A hotheaded character needs to have polite discourse with someone they hate)
Character development! It’s important to actually show your character’s personalities and abilities, rather than just tell the readers that they have this particular ability or fault or whatever.
Don’t tell us that your character likes to read. Show us their endless bookcases. Show us their stack of re-read favorites. Show us them rambling about how Edgar Allan Poe changed their life.
This is the same as it should be with faults. Don’t tell us that your character is not physically fit. Show us them needing to catch their breath after a sprint. Show us them struggling to get even a single pull-up. Show us their flushed cheeks and wobbling legs after a game of dodge ball.