If an actor should deliver his or her lines in a particular way, a screenplay will contain a description in parentheses to illustrate the point. Parentheticals should be used only in cases where a line of dialog should be read in some way contrary to logic. If used too often, actor's and director's egos get hurt, and things get messy.
For Example:
JULIE
(calmly)
I hate you. I hate you. I hate you.