Not by just knowing your lines but by also understanding your lines, word by word. Every word was written for a reason. What does the word describe? What does it mean and what does it mean to the person you are telling it to?
Why are you telling it? Who is the one you are telling it to and what does it do to you when you say it? How do you feel about him/her and what do you want from them? Which moment do you pick to say it? Where did you come from, what happened before and where are you going to?
When you know and feel all that.. then just let it happen. Do not think about the lines anymore because you know them! Let it go. Let it happen to you because you are the character.
You don't act a character, you ARE the character! Inside and out!
View some auditions for "Games Of Thrones"! It includes a good example by Carice van Houten to show that it does not always go as planned and that it is ok to make mistakes. But how you cope with them is what matters. Are you prepared? Stay relaxed. You know you can do it! As soon as you start to worry, you make it impossible for yourself.