Well as the story goes; the medium said; " I saw a spirit ascending out of the earth"

When she was ask to bring up the prophet Samual...  Then she was ask what form the spirit had??  After she described the spirit; {which must not have been visible to King Saul} he perceived it was Samual...   Then of course the followed the exchange of words, between Saul and Samual...  Therefore Saul must have heard the spirit speaking....    Note that the spirit seem to have knowledge peculiar to Saul and the  same type intercourse Samuel had used during the prophet's life.... {intercourse has more than one meaning folks... look it up sometime}   

The scripture study guide states: "That Samuel not an apparition, was evident to the eyes of the medium, God allowed the actual spirit of Samuel to speak...  She understood that her inability , to raise the dead in this manner, and she therefore knew it was only by the power of Almighty GOD!!  Amen!