How can one overcome the power of Geass?

Question

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?

Answer 1

According to this wiki page (emphasis mine):

There is only one instance of a person overcoming the effects of Geass, which is the case of Nunnally - Lelouch attributes this to a very strong degree of will power, which may have been helped along by Charles being dead by then, though there is no mention of her recovering the lost memories of her mother's death, which was Charles' "primary" ability.

It is also said that (emphasis mine, again)

Charles' Geass in the anime allowed him to freely alter an individual's memories at will, and also seal the Geass of others, as well as physical abilities such as eyesight, though there are ways to break these seals, such as contact with an Immortal or sheer willpower, though the latter may have only been possible thanks to his death.

So it isn't explicitly stated, but as @Kreiri has mentioned in his comment, the heroic willpower trope also has it's place here.