Writing Adolescent Fiction/Revealing personality
OK, so you've created a character who is arrogant, sociable or inquisitive. But how do you get your reader to surmise that she or he is indeed arrogant, sociable or inquisitive? You want your reader to be able to tell.
The most common (and most pervasive) outlet for giving readers an impression of your characters is through the character's actions. A cruel character will do cruel things. Anything from the most ground-quaking event to the most quotidian reveals character. There is a rule of thumb, don't say "character X is funny", make him do funny things and the reader may realize that. It is the principle of showing instead of telling.
For instance, if you write:
Tina dipped her left pinkie delicately into the finger bowl. Oh, how she loved this dinner!
The reader will get the impression that Tina is dainty and proper.
Having your character tell his mother the truth when she asks what he did on a perfectly ordinary school day does not build the impression that he is an honest character, as only a person suffering from problems with confabulation would find any reason to lie about that. However, suppose you had your character turn in missing tickets she found:
Her arm shaking, Rachel took one last look at the concert tickets. "They're all yours", she said quietly.
"Oh, thank you once again," the woman with the blue purse said, falling all over Rachel. "I don't know how I'm ever going to repay you."
"I really wanted to see Fourth Ear Deaf in concert." Rachel looked down. "But these tickets weren't really mine."
"Girl", said the woman, "To see someone with the integrity of you made my day. That will bring me more joy than seeing any concert."
The woman took the tickets from Rachel's hand and put them safely in her purse. She crumpled her hands up, thinking about the upcoming Fourth Ear Deaf concert. Rachel gave one last wistful glare, then turned away.
Then you (and your character) have really impressed the reader!
If you only write parts that advance the plot, you will have to have the characters' personalities be really central to the major events -- they must be behind these events -- if your characters are to come across in the reading. Including chapters that focus more on the everyday life and personality of the characters -- the "packing peanuts" -- gives you more flexibility, as you get more chances to develop every aspect of a character. Using the example of a cruel character doing cruel things, in the major plot-moving sections you can have your character betray another, or even kill off the protagonist at the end. In the more mundane sections, you can have your character stick dynamite up a cat's rectum right before he meets another character in a dark alley, or push over a disabled student on a school day, or crack a tasteless, heartless joke during a conversation at the hamburger joint.
Main chapter: Dialogue
In real life, you draw some major conclusions about the people you meet by listening to what they say. So will your reader draw conclusions from reading what your fictional characters say. Suppose you are reading a novel and hear Mindy speaking:
"Let's go!", said Mindy. "Take your sticks and beat Buena Vista over the head! Come on! Come on!"
Noelle rubbed the end of her lacrosse stick, glanced up and Mindy and looked back down. "Wish me luck."
"Luck? We're not going to need luck. We're going to win by our force and strength. What are you waiting for?"
You get the idea that Mindy is loud, aggressive, a go-getter and much like a sergeant in the army.
Almost every sentence a character speaks can give you some idea of who the character is. A lot of television shows, in order to save time (after all, they only have 26 minutes), will have a character slam down the telephone without saying good-bye. "Saving time" like this is a bad idea in written fiction. Saying "Good-bye" after completing a phone call is such an ingrained convention observed by the vast majority of people that not saying good-bye would be a stronger indication of the type of person a character is than actually saying good-bye would.
Word choice 
The power of dialogue doesn't stop at the sentiments expressed in the character's sentences. The very words themselves can reveal something about his character. Consider the differences between the following characters:
"I saw Ben pooping under the tree in the backlot", said Chris.
"I saw Ben defecating under the tree in the backlot", said Trav.
"I saw Ben taking a shit under the tree in the backlot", said Mark.
"I saw Ben leaving his memoirs under the tree in the backlot", said Denzel.
"I saw Ben shaking his pepper shaker under the tree in the backlot", said Miguel.
"I saw Ben going poo-poo under the tree in the backlot", said Billy.
See Dialogue for more on the power of word choice.
In addition to the character's words, the things the character thinks but does not say can reveal him or her.
Suppose your character Natalie sees a boy walking by and thinks the following:
Natalie looked appraisingly at Josh and then stopped there and stared. As if!, she thought. Is this guy even aware that a choker necklace is dorky and so not hip? I am so not going to date him.
Natalie has not said a word so far, yet the reader will already be sure that Natalie is not only fashion-conscious, but superficial.
When the writer uses this technique extensively, it is called Stream of consciousness. Some examples are Ulysses, by Joyce; Hopscotch, by Julio Cortázar.
We get our first impressions of people we see by how they are dressed, and so will your reader.
How neatly a character is dressed can provide instant clues as to his or her personality. If Jerry has the top three buttons on his shirt unbuttoned and his baseball cap on at a totally odd angle, with his jacket fit loosely on his back, the reader will conclude that Jerry is rather slipshod. You plant in the reader, among other things, that Jerry does not pay attention to small details and would likely not get a good grade on the upcoming bio quiz on binomial nomenclature. If Phil's shirt is tucked in, on the other hand, your readers will expect Phil to be meticulously conventional. If your character is kind of flaky or if your character has ADD, perhaps she will come to school one day with her sweater inside-out.
The kind of clothes someone is wearing provide a lot of information about who they are. They can identify what clique your character belongs to: someone with a black trenchcoat, black eye make-up and a pentagram necklace could be identified as a goth. Someone who wears a beret to school would probably be artistic. A rugby shirt, or a baby doll dress, or capris, or a ra-ra skirt, or a cowboy hat, or wallet chains all have their associations, and bring certain personality traits to mind.
How the outfit is put together can also give an impression of a person. Someone who wears green sweatpants with a puce sweatshirt is not the same kind of person as someone who comes in blue chinos, white shirt and beige jacket. Often writers, cartoonists or screenwriters show that someone is the absent-minded professor type by having him wear mismatched socks, but this has become a cliché and you should avoid it.
Having your character change and dress differently on two occasions can also reveal something about him or her:
Cole locked the door and started to throw off his black Chromica T-shirt. About to start class in less than a half-hour, be figured he better throw on a red polo shirt. Then he opened his closet where a cashmere sweater was folded neatly. He put it on over his polo shirt and looked into the mirror.
Cole seems rather like a plastic metalhead. A character who wore the polo shirt and cashmere sweater all the time, or who wore Chromica T-shirts to school, would be quite a different kind of person.
Likes and dislikes 
Who doesn't like chocolate ice-cream? Such a statement is used to express how good chocolate ice-cream is, but questions like this can help construct a character's image or personality. If your character likes ice-cream, sure it is helpful to establish that they are rather average. But if a character is a pessimist or more morose or dark character, they may say "Ice-cream? Blech, no. Too sweet." People have just learned something about this character. A character who like football, basketball & jogging obviously likes exercise. A character who is fat obviously won't enjoy such things. One of the things you must remember is that people don't do things that they don't want to do. Even things they don't really want to do, like homework and doing the dishes, they are really choosing the lesser of two evils (they choose to do their chores rather than be punished) so what a character likes gives a little insight into what a character does, or wants to do. Now, of course, you can run at it like a neanderthal and say: Suzie likes cooking and puppy dogs. But both you and the reader will have more fun if you have Suzie cook something and express her delight at doing so, then give the dog the bowl to lick, all the while smiling from ear to ear. Very important however, is that you really shouldn't mention a 'like' unless you are going to write about it. Your readers will feel short-changed if you mention that Gary likes sky-diving, and then never ever mention sky-diving again.
With dislikes however, don't get confused with fear. When I say I hate spiders, I mean that I am afraid of them. If I mention that a character is afraid of spiders, it may be interpretted as a chekov's gun, and people will expect my character to be confronted by spiders at some point. Also, if John hates Ashley, then this is also not dislike, this is rivalry. If John is the protagonist, people will expect Ashley to be the antagonist, if not an obstacle. These can however, both be interpretted in certain ways. Perhaps a fear of spiders is a way of showing that a character is afraid of silly things, and as such is cowardly. Perhaps John hating Ashley shows how John is an aggressive character. Just be careful that you aren't saying something that you don't mean. Otherwise, dislikes are very similar to likes. If Peter dislikes reading, it shows that he is probably a more outgoing or active character than Benjamin, the little bookworm who loves reading science fiction stories.
Which brings us to the main and probably most important feature of likes and dislikes. And that is the question "Why?". Jeremy likes to eat potato chips, but dislikes dogs. This can say something of a character, perhaps Jeremy is lazy, or doesn't like healthy food. Perhaps he is too irresponsible and hates the idea of looking after an animal. You can learn something here. However, what if I told you that Jeremy likes potato chips, because when he was younger he and his father used to sit on the couch, eating potato chips, watching action films. This shows that Jeremy likes his father. Liking potato chips is just one facet of this, and there may be many others. What if Jeremy dislikes dogs, because as a child he was bitten by a dog? Perhaps he is traumatised, or maybe he has scars. This all adds up to what a character is. Think about what you like and dislike and why. Knowing such things helps to construct characters
What other characters say about him or her 
Describing directly 
Of course, there is always outright telling the reader what your character's personality is like:
Fred was masculine, short-spoken and dependable.
Even when writers do this, their characters' personalities usually shine through anyway through their actions, dialogue, etc. In fact, there is not much use to this direct characterization, as it is called, since if you do also show a character trait through actions or dialogue, describing the character becomes redundant, and if you don't show a personality trait anywhere in the story, the trait is probably not worth mentioning.
However, do use direct characterization if your characteristic is so impressionistic that no indirect characterization can perfectly convey it:
Mandelyn was a light, kelp-free wave briefly visiting upon the shore, with all the simple but mysterious elegance of a Japanese rock garden.
A final word 
If you have some packing peanuts in your story -- a chapter or part of a chapter that does not move the plot along -- make the best of it and write parts that really show what your characters are like. A dialogue between two different characters, or even two ostensibly similar characters, can bring out the differences in their personality.