In William Shakespeare's play, Romeo and Juliet, characters influence the action of the play. Lady Capulet is a rather young wife to Lord Capulet with hasty reactions to circumstances. For example, when Romeo kills her nephew, Tybalt, Lady Capulet demands that the Prince have Romeo executed in revenge. If she would have stopped to think, she would have recognized that Romeo dying would have escalated the feud to intolerable levels with much of the city at war. She also lives at a time in history where the wife really has no say; the husband rules and makes all the decisions. Lady Capulet doesn't support Juliet in her reluctance to wed Paris, but indeed tries to persuade Juliet to follow her father's wishes. The advice I would give to an actress playing Lady Capulet would be to first put herself in the frame of mind that the husband rules all, and she must appear to truly believe that. I would also help the actress remember that she is portraying a very young wife and mother with a rather nasty temper. The actress needs to portray the wife of Lord Capulet as a person with prestige in the city who really doesn't know her daughter. Her daughter really doesn't trust her mother with any real feelings or information which says that her daughter gives her affection to the nurse. The last thing I would remind the actress is that Lady Montague dies of grief, but Lady Capulet really only changes at seeing Juliet dead in the tomb which allows or forces her to agree to the truce between the two warring families. The actress has to be prepared to make that dramatic change from the woman who demands revenge despite the harm it would create in the city to a woman willing to make peace because death has finally touched her with the death of her daughter, Juliet.
Just some general Shakespeare advice...
Read aloud and in front of the mirror often. Analyze the iambic pentameter to get a feel for where stresses would be appropriate. There should be heavy markups and highlighting in addition to stage directions. Connect emotionally to your character and identify her motives. What does she want from herself, from others?
From personal experience, acting in a Shakespeare play is incredibly rewarding. Break a leg!
Theather is all about becoming your character. I would sugest to watch the movies and take note of lady capulet's actions and tone. Also, research the time period to understand more on how she would think and understand. Use and practice an accent if required. Also, understand her place in the social hierarchy.
If the play is being done with accents, my biggest piece of advice would be to practice the accent and make sure every word is clear and enunciated. Also, she should be aware of things like her posture and the way she walks. The Capulets were royal, which means Lady Capulet would have been trained to walk and stand with a straight back.