Stay in character.
It is very important to stay in character. Often, if someone says you're being OOC(Out of Character) they may not be refering to the talking you are doing.
For example; Draco would not apologize to Hermione for any reason under the sun, unless a professor was beating down his neck. Harry is strong and is not going to cut himself. Do not turn a good character into a bad one just becaue you think they are ___.
Two users have some
very good ideas. I highly suggest you read the following.
Ryuuno Haruka
One. Do a bit of research for your character. You probably don't need to go into the detail I do for some of mine, but knowing some of what your character should know is helpful. For example if you're playing a military character go look up the character's branch's General orders and what the various ranks are called.
. . .
Two. Please be logical about your character's race.
These are two very important points. If you know nothing about Quidditch, then why be on the team? If you don't know any spells -- then don't challenge someone to a duel.
And please. Do not. Be a Mary Sue.
TANSTAAFL
Stay in the rules. If it is the Potterverse, then you need a wand. Your new kid isn't going to have invented a new spell that can paralise people with just a hand gesture. Remember, it takes time to learn anything, so a ten year old will have less spells (goes for martial arts, book smarts, etc. as well) than a teacher. Maybe more raw power, but less control and ability to channel it. So you won't be beating on any adults with your twelve year old any time soon.
Here, he restates the importance of common sense and logic. Play fairly and don't make your character x 10 better than everyone else. 'Cause you know, that's like so not cool.
How do I stay in character?
Now that you know about staying in character, the following lesson will teach you how to become in character and how to stay as it.
You need a piece of paper and a pen. Write down the normal things about your character(name, year, house, etc,) but then write down traits. Is your character a nasty Slytherin or a shy Ravenclaw?
If your character has a bold demeanor, try to keep in mind that the character will never turn down a fight, but will most likely antagonize it.
If your character is shy, then keep the speaking part to a minimum. Let your hands and eyes do the talking.