His monologue from the movie (and scene):
Parry: It begins with the king as a boy, having to spend the night alone in the forest to prove his courage so he can become king. Now while he is spending the night alone he’s visited by a sacred vision. Out of the fire appears the holy grail, symbol of God’s divine grace. And a voice said to the boy, “You shall be keeper of the grail so that it may heal the hearts of men.” But the boy was blinded by greater visions of a life filled with power and glory and beauty. And in this state of radical amazement he felt for a brief moment not like a boy, but invincible, like God, so he reached into the fire to take the grail, and the grail vanished, leaving him with his hand in the fire to be terribly wounded. Now as this boy grew older, his wound grew deeper. Until one day, life for him lost its reason. He had no faith in any man, not even himself. He couldn’t love or feel loved. He was sick with experience. He began to die. One day a fool wandered into the castle and found the king alone. And being a fool, he was simple minded, he didn’t see a king. He only saw a man alone and in pain. And he asked the king, “What ails you friend?” The king replied, “I’m thirsty. I need some water to cool my throat”. So the fool took a cup from beside his bed, filled it with water and handed it to the king. As the king began to drink, he realized his wound was healed. He looked in his hands and there was the holy grail, that which he sought all of his life. And he turned to the fool and said with amazement, “How can you find that which my brightest and bravest could not?” And the fool replied, “I don’t know. I only knew that you were thirsty.”
UPDATE: You can now listen to Terry Gilliam’s commentary for The Fisher King online.
Sadly, it was never on the DVD; only laser disk…