Tropes and Cliches

Character Development: In General

Ever notice how a character went from the Straight Man to the Comical Relief in a span of one season? Or how that one character that was sunshine personified become the spawn of Satan within 2 and a half books? Let's talk Character Development.
Now Character Development isn't something that happens overnight. It takes time. But what is Character Development? Well it depends on the context of the situation. By definition, Character Development is the process of creating fictional characters with the same depth and complexity as real-life human beings. Any of the characters of a book or movie can change for better or worse in the span of the piece of media we're looking at. And that change can be any thing really. 
Let's look at  Zuko from Avatar: The Last Airbender. Probably the most talked about. His development starts out as an angry, exiled prince, who's only way home is to capture and bring back the avatar. Someone no one has even seen or heard of in over 100 years. Everything hinged on bringing home someone who was over 100 years old or the head of a water bender. By season 2, he and his uncle ( probably his only Moral Compass, which is for another time) are on the run from the very nation that exile him, while getting a bigger picture of who he was via Flashbacks and character interaction, with him getting sick after doing the right thing but fumbling. Season 3, he's still struggling before accepting that his philosophy and morality has shifted and helped end the Fire Nation's genocide and war. And this is just the cliff notes. 

Getting from A to B is always a ride for a lot of characters. In some cases we get the development in reverse. Rose Quartz, for example, in Steven Universe  was shown to be this caring, curious being fighting for the Earth. And when it was revealed ( something everyone and their moms' called) she was Pink Diamond, the same being that sought the same power as her sisters, and had a temper on her, it was hard to continue seeing her this kind person. 

So how does one develop a character? Well, what are we looking to do? One have to consider the story you are trying to create. Is it some sort of coming of age or some type of journey to find a MacGuffin? What traits does the main character have? 
In some case you can see it in how they interact with the world around them. For example, we can see the Damsel character go from being helpless to hitting the problem super hard. Or the guy that needs to be the center of attention 24/7 have humility and knowing when to step back.  
In fact, Young Justice gives us a taste of Character Development after the simulation that was supposed to go wrong...go wrong in the worst possible way. You can see the shift in everyone. Everyone admits to something that was bothering them, yet you can see the very subtle changes. 

Also, try to avoid making that one change their whole personality. There's a trope for that and I will discuss it in the future.

Character Development isn't a one stop shop. Characters grow and change as their story continues. It's doesn't have to be from Point A to Point B, we can see the change and be reminded of who they were in the past as many Mentor characters have "A PAST". We can see characters go from Holier than Thou to Hellspawn. It just depends on the story you want to write.