God is no trickster. He doesn't try to fool us into doing His will. He wants our full assent knowing how difficult conforming our will to His may be. So how much did Gabriel reveal to Mary about the suffering she would share with her Son? Fulton Sheen believed that Mary knew the depth of pain and grief she was choosing. Here's how he describes it:
Mary’s participation in Christ’s suffering began with the annunciation, when she was asked to give God a human body, more properly, a human nature. In other words, will you make God capable of suffering? God though he was, he learned obedience in the school of suffering. God could know experimentally what suffering was only by taking a body. So the Blessed Mother is asked, ‘Will you make it possible for your creator to suffer?’ Think of a mother, for example, who gives to a young son or daughter an automobile at the age of nineteen, which a short time afterwards is the cause of a wreck and permanent injury. Would the mother ever forgive herself? And here Mary has to say yes, I will let him suffer.

