Analyzing the Play's Text
Read through the script once to get an understanding of the story and the different characters, while highlighting all of your lines. Then go through each of your lines from the beginning to analyze them. This includes analyzing the character's different emotions, including different layers of emotions, such as those she shows on the outside and those she actually feels on the inside. It also includes analyzing how this character would behave. What mannerisms, gestures and other nonverbal expressions would she have while saying each line? Make notes in your script. As you practice your scenes with your ensemble, make notes of these ideas on your script also, as it could change or be affected by your character's interactions with other characters. Your director most likely has an interpretation of your character in mind, so you can refer to him if you're having trouble with ideas or confused about the meanings of any of your lines.
Experience Your Character's Emotions
When practicing with your ensemble, instead of just reading your lines and doing the gestures, try to actually experience your character's emotions. Try to empathize with your character by thinking about her experiences and her emotional state. Imagine that you are her. Think about whether you have a personal past experience similar to your character's. When portraying your character's emotions, it can help to make a connection between your past experiences and the character's experience. Remembering a traumatic event or exciting experience from your memory while portraying your character can help you portray emotions more believably.
Physicalities
To become a new character, your body must fit that character's age and personality, and reflect the person's emotions in each scene. For example, if your character is feeling extremely stressed in one scene, your body should reflect that, perhaps with slouched shoulders, jumpiness, and an anxious or worried countenance. In the next scene your character may be intensely happy and your body would instead reflect a feeling of lightness and your face expressing a feeling of joy.
Bring Energy and Life into the Character
Believability and realism stems from energy. You need to breathe life into your character by putting your body's adrenaline and endorphins into your acting. Try to feel like an empty vessel that you fill up with your character. Aim for an energy shift that causes barriers holding back expression to be broken down. You should experience a flow of intensity different from when you live your everyday life.
Mark Your Script and Memorize Your Lines
Mark in your gestures, sighs and other nonverbal cues in your script. Use your script to study your lines often, both alone and with other actors. It's important to spend enough time with the other actors to obtain a believable chemistry with the other characters. Ask for feedback from the director or other actors about your performance to see how you can improve the portrayal of your character. Work towards memorizing your lines and nonverbal cues so that you can act without the help of your script.