Humans are Complicated..
No matter how bland, slow, dumb, lazy, or cruel humans are very complex

1. Do I think I'm complex?
No
2. I'm actually complex?
Probably yes

So when writing characters, there so much of the human psyche you can explore and you don't have to be a psychology major to write a good character

I'm gonna use an example: forest grump, now in the movie he seem quite simple but even though Contrary to his nature forest show more emotion then some characters

He did things, that no one thought he could forest join the war, he continued on a dream for his friend, ran across several states for many days, he even searched and protected jenny

You went along with him, experiencing the feeling he felt and the things he lived

Forest is an actual likable human being

That where author fail sometimes, you don't create humans you make one Dimensional "people"