How your character views their surroundings can be very revealing. A paragraph of beautiful prose describing the leafy trees and a smooth glassy lake might not work best for a character with a sour or sarcastic disposition. Really think about how they see the world and apply it to everything from a classroom to a castle (if they're going inside a castle, of course).
From what they choose for lunch to how they react in a crisis, the depth and length that they debate choices can do a lot to help show a character's voice. Always stay true to your character when plotting. A stubborn character's will can't bend suddenly because you need to get from A to B on your outline. Choices can show character growth (or decline) but can't come from left field.
When your character first walks into a room, what's the first thing they notice? The atmosphere? The other people? The colors of the floor tiles, for the more shy and reserved?
Knowing how your character's mind works and really stepping into their shoes for each and every scene helps create solid and believable characters.