This dialogue touched on issues of God's nature and physical manifestation. Memorizing verses sure does have its advantages. We do not know what spirit is, but we do know what it is not. Also, God is everywhere. A body would mean that he has material, Matter, etc.
Time is a function of the existence of Matter.
Since God is outside of time, eternal, He could not be material. Therefore, He cannot have a body because he could not them be omnipresent. Not sure really. But God reveals Himself to those who seek Him. With Stephen maybe it was because he was the first Christian martyr.