Albert Barnes Commentary 1 Samuel 28:12

"And when the woman saw Samuel, she cried with a loud voice; and the woman spake to Saul, saying, Why hast thou deceived me? for thou art Saul." — 1 Samuel 28:12 (ASV)

It is clear both that Samuel's appearance was real and that the woman was utterly unprepared for it.

Why have you deceived me... — She perhaps inferred that Samuel would have answered the call of no one less than the king. Or it may be that the presence of an inhabitant of the world of spirits brought a sudden illumination to her mind.