Read the sides. This may or may not be the entire scene in the script. Before you find your character, examine the scene for detail. Think of why the scene is in the script. Think of what page number it is and what that means in terms of the overall screenplay structure. Interpret the intention of the scene. Imagine what the close-ups are in the scene and how they might be filmed. Study the pacing of the scene.
Build your character based on the sides. If the scene is well written you should be able to devise some characterization from it. However, stay open to the ideas of the director.
Listen to the instruction give by the director. It is the director's vision that makes the movie and he is the one who will be directing you, should you be cast in the role.
Relax and be yourself. In addition to playing the character, your personality should come through in the role. The more comfortable you are in the casting session the better your reading will be.
Do not try to memorize the lines. Being the character is more important than trying to play the character as written. Take the instruction given by the director and deliver the lines as best as you know them. Do not try to remain faithful to the lines if you feel it will get in the way of your performance.
Keep your motivation during the reading. Believe in your character. The director wants to see you make choices in the performance. Know where your character is supposed to be in the scene. Know why your character is acting the way he is. Know how the character is feeling. Most of all, understand the psychology of your character and what his driving motivation is.