How should an actor change Romeo and Juliets tone and pitch?

This is really a question about acting, not about Romeo and Juliet. All actors must use variations in tone and pitch when playing any part, because it is these musical effects (as well as rhythm, tempo and volume) which convey the emotional or affective content of the line. Of these, tone is probably the least variable: an actor will adopt a tone for a character, usually one not far from the natural tone of his own voice, and stick with it unless the character's voice is under stress (due to illness, repressed weeping or laughter, having a hand on the throat, etc.) or if the character is stepping out of his own character to imitate another (Robin Williams does this all the time).

Pitch is used to emphasize as well as to define emotions. Some actors of about a century ago, when delivering a long speech from Shakespeare, would deliberately set the pitch at a certain value at the beginning of the speech and raise it a semitone periodically as the speech went on to indicate increasing excitement. Modern actors rarely use it as broadly and overtly as that, but it is crucial in determining how to deliver a line.

For example, "What light through yonder window breaks?" can be delivered starting on a high pitch, coming down on light, and continuing until the Word "breaks" when it rises again. But if you start lower on "what" raise the pitch on "light" it emphasizes "light". If you raise the pitch on the syllable "win" of "window" it is natural also to linger on the Word, emphasizing window, and so on.