Have your characters react to each other. If you have one character say the other is a jerk while the other is doing something absolutely innocent, for example, that will say something much bigger about both of the characters.
Think about it this way: if you only just met someone and they told you, "I'm the best person you're ever going to meet. I'm so nice," would you really believe them? Probably not. But if their friend walks up and tells you a story about how this guy saved someone's life once, your opinion of that person would likely go up.
Consider how you can use various reactions to your benefit, particularly when your story starts.