In comparing the audience's sympathy for Medea in Seneca's and Euripidis' versions, what is the importance of her talking or not on stage with others?
In Euripides's version, Medea is offstage as the work opens up and her difficulties are communicated through her nurse. Medea is alone and she is weeping in anguish over what has happened to her. This is effective in creating a sense of pathos for two reasons.
First, when someone else speaks in your stead, there is a greater sense of transparency, honesty, and modesty. To be sure, this is not always the case, but often when someone communicates your plight for you, it give the impression that the one who is speaking is a more objective communicator.
Second, by having Medea offstage (perhaps only hearing her wails), she is characterized as passive. Tragedies have been done to her. She is a victim of the gods. She does not even have a voice. All of this creates pathos.