Tools of Characterization
Characterization in Finding Nemo
Family Life
Finding Nemo's all about a father and son, so you can bet family relationships are one big way that we learn about the characters. Marlin's totally haunted by the memory of his wife's death and so he's super-overprotective of the only family he's got left.
Nemo, for his part, resents the way his dad is always trying to hold him back. That's the whole reason that he swims out into open water and gets caught:
NEMO: I can swim fine, Dad, okay?
MARLIN: No, it's not okay. You shouldn't be anywhere near here. Okay, I was right. You'll start school in a year or two.
Oof. That's worse than the worse dad joke.
But, Marlin and Nemo's are the only characters with family issues. Dory also has a complicated relationship with her family—she can't remember them. The members of the Tank Gang have no family either. They all came from pet stores and websites. Even Bruce the shark never knew his father. It's enough to make a great white cry.
Okay, but having or not having strong family bonds isn't the be-all end-all in this movie. It's clear that fish can learn and change and strengthen their family ties—it's one of the ways we see Marlin and Nemo grow as characters.
And as for those fishy folks that don't have family—they form lasting surrogate families (just take a look at the Tank Gang). It just goes to show you how important those connections are. People need people and fish need fish.
Location
The ocean's a beautiful place. It's also a super dangerous place. Marlin learns this in the very first scene of the movie. He might have scored an awesome anemone home on in the Great Barrier Reef, but that alone can't protect his wife and children.
And life away from the reef isn't super safe either. Marlin's actually kind of smart to be afraid of things like sharks, jellyfish, and swirling vortexes of terror.
The dentist's office, on the other hand, is more secure than the ocean (no sharks swimming around in there), but it's not exactly a happy home:
GILL: My first escape, landed on dental tools. I was aiming for the toilet.
NEMO: Toilet?
GILL: All drains lead to the ocean, kid.
NEMO: Wow. How many times have you tried to get out?
GILL: Ah, I've lost count. Fish aren't meant to be in a box, kid. It does things to you.
The ocean might be scary, but it also means freedom. You can swim around, enjoy the natural wonders…and you never have to worry about going home with Darla. No matter what location they're in, it's pretty clear all our characters are longing to be free out in the deep blue sea. After all, there's no place like home.
Speech and Dialogue
The way you talk says a lot about you. Just take Crush, for example. We can tell by his laid-back surfer speak that he's one chilled out turtle (maybe that's why he's such a cool dad):
CRUSH: Oh, saw the whole thing, Dude. First you were like, "whoa!" And then we were all like, "whoa!" And then you were like, "whoa."
MARLIN: What're you talking about?
CRUSH: You, mini-man. Takin' on the jellies. You got serious thrill issues, Dude.
[…]
MARLIN: So, Mr. Turtle...
CRUSH: Whoa, Dude. Mr. Turtle is my father. Name's Crush.
Dory has a frantic pace to her dialogue and she likes to changes subjects quickly (we think we remember something about her having a bad memory). Marlin's voice just drips with anxiety and unease. Nemo is young and chipper and optimistic when he talks. And Mr. Ray loves to make up crazy songs.
One weird thing is—for a movie set in Australia—the dentist is one of the only characters with an Australian accent. It's a little weird, but it sort of sets him off from the tiny fish characters. This human is a product of his environment, but the fish exist outside that world.