In Code Geass R2 episode 49: "The Grip of Damocles"
Lelouch confronts Nunnally, who reveals she has regained her sight by overcoming the power of geass that was cast by her father King Charles..
through will power.
What kind of will power would it take for her to overcome her father's geass? Did she has to take intensive training or such to do so? How can one overcome the power of geass that was cast on him/her without the usage of Jeremiah Gottwald's geass canceller? Or the geass would automatically be cancelled upon the caster's death?