What is it with you people? You think not getting caught in a lie is the same thing as telling the truth?
Lastly get emotionally connected to your story so you can deliver it, you know, if you can't deliver the emotions to your script there's no point to your story. Story is the key.
Usually I like to improvise. Sometimes, depending on the nature of the piece, I like to improvise because I think it brings a certain freshness and a reality to it, as long as it doesn't go too far out of the box.