You can check out part 1 - Show don’t tell if you find this one interesting.
This post is going to be about character development.
What makes some characters so memorable? Lovable even. And why can the wind blow straight through our ears and take other characters away? Never to be remembered again.
I’m jotting down three reasons I have thought of. You can share any of you can think of too, or expand on them in the comments.

People don’t change, they develop

This may sound like splitting hairs. Because it is. To change or to develop? Same thing. But, the point still stands. Though we can become different people over time. Even feel brand new. The fact remains that wherever you go, there you are. It’s still you with all your experiences in there. If there is a fundamental you, it's you with all your experiences of the world from your perspective that cannot be reproduced. The you of yesterday is still inside the you of today. It’s just that you have developed on top of yourself. Added a layer. Same applies to great stories.
Movies really lose me when a character does something “out of character”. Even if they are going to do something that a character like them wouldn’t do. Say, a timid character doing something brave. If they don’t do the brave thing timidly, it feels wrong. This is what makes Chihiro from Spirited Away so believable. She is on this scary, complicated, dream-like adventure, she is acting bravely, but everything she does still feels like she is a scared 10 year old girl who is looking for her parents. And this helps make the whole story feel real though it is so surreal.

No one talks the same

No one talks the same, so characters in a movie shouldn't either.
If you been in any meaningful relationship, one of the most obvious problems is communication. No one talks the same. We all communicate differently. We often talk past, at, or around each other. Even when communicating honestly and openly, it can be a total circus of back and forth, you saids, and that’s not what I meants. While putting a 100% realistic conversation on screen would be simultaneously mind numbing and aggravating, good dialogue gives each character their unique voice and position. The diner scene from Reservoir Dogs comes to mind. Or, any scene from In Bruges. Wes Anderson’s dialogue is very stylized and tight, which should make it hard to connect with the characters. But, he still makes each character sound and feel unique.

Give me motives, not movement

What really kills a movie for me is when characters have such a lack of character that what they do doesn’t even matter. You have no clue what their motivation is, so they can do anything, say anything. A true NPC situation. Great movies make even the small characters have clear motives. A great example is It’s a Wonderful Life when Uncle Billy loses the money at the bank. It’s one of the most critical parts of the movie, and Uncle Billy is just a side character. But, they establish his character so well that when he loses the money, you groan inside. It feels inevitable. Of course he did that. What are we going to do now?! Without his character being so subtly, but expertly, develop, the money being lost would have mattered for the plot, but not to the audience. And this scene would make George just seem like a jerk.
Alright. Those are my three observations on what makes great characters in a movie. What else is there?