Read the entire play from beginning to end several times so you know not only the story, but also the place your character takes the scheme of things. It's also important to understand the other characters in the play so you know how to interact with them.
Memorize your lines. Do not use inflections or try to give them character in any way at this point. Simply read and reread your lines until you can say them backwards and forwards (preferably forwards), because when you're onstage there is plenty of pressure that can make you nervous, and you must have the lines firmly embedded in your memory.
Create a history for your character. Once you understand your place in the production you must understand how and why the character acts and speaks. Take a notepad and write a biography of the character as you see it, always taking into account the interactions with other characters. This can help you authenticate your performance.
Become the character when rehearsing the role. Do not act it out; make the character your own and rely on your understanding of the character and his purpose and intent during the play. If you have learned your lines and believe in the character you've created you'll portray the part believably.
Interact honestly with the other characters. If you're comfortable in your character's skin, the actions, inflections and nuances tend to come naturally and you'll react to the other characters without needing to create gestures and expressions; they will come quite naturally and the audience will believe in the character you've developed.